things to look for, considering Makita, Hitachi, Milwaukee, Bosch all have a great product (even Dewalt is suppose):
Battery cost/ replacement
Battery Ah, most cheaper models have 1.5ah, longer lasting have 3.0ah!
LED lights
Clutches and speeds, especially impacts
Weight
Warranty
Hitachi has a 10yr warranty on all their Li-on tools now, Ridgid has a Lifetime but its very specific to how you can actually receive it.
Personally I think the batteries are the biggest issue, how affordable/ popular are the batteries and do they have a warranty? The tools are all very effective at what they do, give you get the top brands.
I have a impact and clutched drill from Hitachi that has been amazing for over 3 yrs heavy use. The 3.0ah batteries are just now starting to loose full charge. Replacements are out there, but Hitachi changed design to a "slide" style, not a traditional bottom snap in battery = hard to find $75+ batteries
check out this site, great prices on Makita, Hitachi, +
www.bigskytools.com