Taken from
http://www.fas.org/man/dod-101/sys/ac/f-15.htm
F-15 C
The F-15C is an improved version of the original F-15A single-seat air superiority fighter. Additions incorporated in the F-15C include upgrades to avionics as well as increased internal fuel capacity and a higher allowable gross takeoff weight. The single-seat F-15C and two-seat F-15D models entered the Air Force inventory beginning in 1979. Kadena Air Base, Japan, received the first F-15C in September 1979. These new models have Production Eagle Package (PEP 2000) improvements, including 2,000 pounds (900 kilograms) of additional internal fuel, provision for carrying exterior conformal fuel tanks and increased maximum takeoff weight of up to 68,000 pounds (30,600 kilograms). Externally, the differences between the F-15A and F-15C are so slight as to make identification difficult; the only reliable indicator is the aircraft serial number. All F-15As have tail numbers starting with 73- through 77-, while F-15Cs have tail numbers beginning with 78- through 86-. The F-15C is the Air Force's primary air superiority fighter, serving with active duty units at Langley AFB, VA, Eglin AFB, FL, Mountain Home AFB, ID, Elmendorf AFB, AK, Tyndall AFB, FL, Nellis AFB, NV, Spangdahlem AB, Germany, Lakenheath AB, England and Kadena AB, Okinawa. The operational F-15C force structure is approximately 300 aircraft assigned to operational units. In the mid-1990s the F-15C experienced declining reliability indicators, primarily from three subsystems: radar, engines, and secondary structures. A complete retrofit of all three subsystems could be done for less than $3 billion.
The F-15C has an air combat victory ratio of 95-0 making it one of the most effective air superiority aircraft ever developed. The US Air Force claims the F-15C is in several respects inferior to, or at best equal to, the MiG-29, Su-27, Su-35/37, Rafale, and EF-2000, which are variously superior in acceleration, maneuverability, engine thrust, rate of climb, avionics, firepower, radar signature, or range. Although the F-15C and Su-27P series are similar in many categories, the Su-27 can outperform the F-15C at both long and short ranges. In long-range encounters, with its superiorr radar the Su-27 can launch a missile before the F-15C does, so from a purely kinematic standpoint, the Russian fighters outperform the F-15C in the beyond-visual-range fight. The Su-35 phased array radar is superior to the APG-63 Doppler radar in both detection range and tracking capabilities. Additionally, the Su-35 propulsion system increases the aircraft’s maneuverability with thrust vectoring nozzles. Simulations conducted by British Aerospace and the British Defense Research Agency compared the effectiveness of the F-15C, Rafale, EF-2000, and F-22 against the Russian Su-35 armed with active radar missiles similar to the AIM-120 Advanced Medium Range Air-to-Air Missile (AMRAAM). The Rafale achieved a 1:1 kill ratio (1 Su-35 destroyed for each Rafale lost). The EF-2000 kill ratio was 4.5:1 while the F-22 achieved a ratio of 10:1. In stark contrast was the F-15C, losing 1.3 Eagles for each Su-35 destroyed.
F-15 E
Although the slogan of the F-15's original design team was "Not a pound for air-to-ground," the F-15 has long been recognized as having superior potential in the ground attack role. In 1987 this potential was realized in the form of the F-15E Strike Eagle. The mission of the Strike Eagle is as succinct as that of its air-to-air cousin: to put bombs on target. The F-15E is especially configured for the deep strike mission, venturing far behind enemy lines to attack high value targets with a variety of munitions. The Strike Eagle accomplishes this mission by expanding on the capabilities of the air superiority F-15, adding a rear seat WSO (Weapon Systems Operator) crewmember and incorporating an entirely new suite of air-to-ground avionics.
The F-15E is a two seat, two engine dual role fighter capable of speeds up to MACH 2.5. The F-15E performs day and night all weather air-to-air and air-to-ground missions including strategic strike, interdiction, OCA and DCA. Although primarily a deep interdiction platform, the F-15E can also perform CAS and Escort missions. Strike Eagles are equipped with LANTIRN, enhancing night PGM delivery capability. The F-15E outbord and inboard wing stations and the centerline can be load with various armament. The outboard wing hardpoint are unable to carry heavy loads and are assign for ECM pods. The other hardpoints can be employed for various loads but with the use of multiple ejection racks (MERs). Each MER can hold six Mk-82 bombs or "Snakeye" ******** bombs, or six Mk 20 "Rockeye" dispensers, four CBU-52B, CBU- 58B, or CBU-71B dispensers, a single Mk-84 (907 kg) bomb F- 15E can carry also "smart" weapons, CBU-10 laser quided bomb based on the Mk 84 bomb, CBU-12, CBU-15, or another, laser, electro-optical, or infra-red guided bomb (including AGM-G5 "Marerick" air-to-ground) missiles.
Conformal Fuel Tanks were introduced with the F-15C in order to extend the range of the aircraft. The CFTs are carried in pairs and fit closely to the side of the aircraft, with one CFT underneath each wing. By designing the CFT to minimize the effect on aircraft aerodynamics, much lower drag results than if a similar amount of fuel is carried in conventional external fuel tanks. This lower drag translate directly into longer aircraft ranges, a particularly desirable characteristic of a deep strike fighter like the F-15E. As with any system, the use of CFTs on F-15s involves some compromise. The weight and drag of the CFTs (even when empty) degrades aircraft performance when compared to external fuel tanks, which can be jettisoned when needed (CFTs are not jettisonable and can only be downloaded by maintenance crews). As a result, CFTs are typically used in situations where increased range offsets any performance drawbacks. In the case of the F-15E, CFTs allow air-to-ground munitions to be loaded on stations which would otherwise carry external fuel tanks. In general, CFT usage is the norm for F15Es and the exception for F-15C/D's.
The F-15E Strike Eagle’s tactical electronic warfare system [TEWS] is an integrated countermeasures system. Radar, radar jammer, warning receiver and chaff/flare dispenser all work together to detect, identify and counter threats posed by an enemy. For example, if the warning receiver detects a threat before the radar jammer, the warning receiver will inform the jammer of the threat. A Strike Eagle’s TEWS can jam radar systems operating in high frequencies, such as radar used by short-range surface-to-air missiles, antiaircraft artillery and airborne threats. Current improvements to TEWS will enhance the aircraft’s ability to jam enemy radar systems. The addition of new hardware and software, known as Band 1.5, will round out the TEWS capability by jamming threats in mid-to-low frequencies, such as long-range radar systems. The equipment is expected to go into full production sometime in late 1999.
So it's not so much what the F-15 C has as it is what it doesn't have: these Strike Eagles that danced with the Eurofighters were carrying loads of ECM, External Fuel Tanks, and other such drag-inducing features. The F-15C has a 95-0 kill score, thats why it's better at ACM.