They’re two very different games. The original Pacific gives Japan two ways to win. The first way is by capturing and holding a single enemy capital (Western USA, New South Wales or India). The second is by collecting enough victory points to win. The victory point method puts the Allies on a clock to defeat Japan. If they don’t make it, Japan has worn the Allies down into settling for peace without total victory. In this way, Japan wins the game without winning the war. This mechanism can lead to some very tense endgame situations as Japan tries to eke out that last victory point.
If you like a suspenseful contest that may not be decided until the very end, original Pacific is the game you want. If you like a more traditional A&A game, go with 1940.
Marines attack on a 2 during amphibious assaults only (3 when paired with artillery). Other than that, they’re the same as infantry. Only the US gets them.