No one's mentioned it yet, but you can upgrade older units to Infantry; Marines (and Tanks, for that matter) have to be built from scratch. Upgrading costs a lot of gold, but in the late game it should be relatively easy to generate a goodly amount of spare change.
Upgrading provides two main advantages: (a) it preserves promotions, so it's best when used for your uber-promoted units; and (b) it can allow you to modernize your army very quickly, given enough cash.
This second advantage can be extremely beneficial. As soon as you have Assembly Line, you can, potentially, upgrade a large number of Riflemen and Grenadiers (or even older units) to Infantry. That's a big boost in unit strength (14 or 12 to 20) in just one turn. And if they're mostly highly-promoted units as well, then you're just going to pwn the opposition.
A proper amphibious force, in contrast, takes many, many turns to build. If you're facing a reasonably modern enemy, then you need more than just Marines. Your Semper Fi boys will probably need a large amount of supporting units (Transports to carry them, Destroyers and Battleships to protect the Transports, Carriers loaded with Fighters to cause collateral damage before the Marines attack). That's going to take a while, since almost everything will need to be built from scratch.
As for Marines versus Tanks, why not both, if the situation calls for it? I'll use a combination of Tanks, Paratroopers, and Bombers for land-locked cities, and the amphibious force mentioned above to capture coastal targets.