The impac driver will do everything you need, most of them have an adjustable torque setting so you can decrease it for finer work, increase it for batten screws and the like. No dramas there. happy to be corrected but the rotary hammer drill is just that, for masonry etc, it appears to have a quick fit sds chuck, so will only take that style of drill but, won't take an ordinary drill bit or a spade bit, holesaw etc...
Skins are pretty cheap these days, so you are best to have a drill and a rotary hammer drill. As far as batteries go the 3.0 ah will do it but 4.0 or 5.0 ah will give you longer run time, this is important with hammer drills, grinders and saws. The other thing to consider is the range of skins and availability, I run the Milwaukee gear for work and have a fair range of skins, grinders, saws, right angle drills, blowers, fans etc. At the end of the day it comes down to use and budget.
Cheers
Josh