@Wicklic: Nope. They do the same amount of damage (the method call is the same for ICBMs, Tactical Nukes, and Nuclear Plant Meltdown [This is why I ALWAYS build Bomb shelters in a city with a nuclear plant ^^])
HOWEVER, this is the trade-offs between both nukes, and guided missiles:
ICBMs:
Pros: Infinite Range
MASSIVE power increase
Causes devastating collateral damage over 9 tiles, and also levels a fair chunk of a city
Cons: VERY Expensive
75% chance of interception w/ SDI (This is the BIG ONE. It practically makes Nukes entirely useless, as if you get the fact that there is a 1 in 4 chance of an actual explosion, AND the fact that if it DOES work, then bomb shelters will reduce the damage by a FURTHER 50%... it means its better just to build more tanks

)
Tactical Nukes:
Pros: 37.5% Chance of interception w/ SDI (50% chance of avoiding detection is a GOOD thing

)
Quite expensive, but not overly so
Causes devastating collateral damage... blah blah blah
Cons: VERY short range. In fact you really need a sub to REALLY use these effectively, or multiple cities at EXTREMELY close range
Guided Missiles:
Pros: Cannot be countered. Ever. There is no way you can stop a guided missile strike.
EXTREMELY cheap, one of the cheapest milatery units!!
Cons: Weak. No collateral damage, and one-use too. Damage dealt to that one target is minimal.
Short range. I think that it has 1 more range than a Tactical Nuke.
So yeah... thats the pros and cons of the three missiles. I never use Guided Missiles, unless I have TONS of low-prod cities. I also never use ICBMs, as after the SDI, they are just big shiny, worthless, toys.