Unless the squads specify otherwise (Ex: Combat Engineers), most of the american squads are all the same, just different levels of skill. If you don't know, just base your choosing of squads based off the price they cost. The more you pay, the better they'll be.

Some squads are different than others. III there was squad with 3 grenadiers, smg squad. Is there any at squad for US? And what are rangers for?

When I used to play, the US Marines were a great buy considering they came with dynamite. However, I think their squad loadout change. If you really want to know which is the most "cost effective squad", you'll have to lab it out by yourself writing down what each squad comes with and how much the squad costs compared to buying each soldier individually.

If you're the research type of player, this will come easily to you like it did to me. It just requires a lot of hours when you're copying the data. But I don't think this will apply to you, and in general I will say focus more on your in-game tactics.

Cost effective squads will not help you if you do not have your tactics down. I recommend reading this for a good overview: http://www.2ndbn5thmar.com/dm/CCMWorkbookMcBreen2002.pdf