The smaller capacity (2Ah) will provide exactly the same 'kick' as the larger ones (4+Ah), they just won't do it for as long between charges.
The smaller ones take less time to charge (in the same charger) though...
Not entirely true when you get up to the big impact wrenches there is a very noticeable difference and Milwaukee even state the biggest batterys are required to give the quoted torque ratings
Also although 2ah is alright for drills and impacts its a good idea to future proof incase grinders routers chainsaws or the like might be of interest in the future
Even 4ah runs out quickly on a grinder