Genesis port:
speed is slower
Single player. Can choose between both characters. Purportedly have different stats but don't seem to. 
Many details missing (lightning striking poles and creating pits, crumbling background walls, falling bodies from plane) totally gone
Bosses have life bars
first boss is a helicopter instead of a tank.
fourth boss is a tank instead of a gigantic crane
Additional boss before final boss - gigantic mecha
Both vehicle sections are missing. However, the level introduction text was not changed. it says "Chase the submarine" but you begin right on top of the submarine already. Jeep is gone.
Final base had obstacles like electrified floors and spiked ceilings, these are missing in Genesis version
Playable but very stripped down