Kinda depends what you're doing with it. For serious, heavy duty work...Dewalt. 18v or higher. Milwaukee makes a slightly better quality tool, but the higher price doesn't usually justify the minor difference. The impact is just about the best invention they've come up with in a while. We only use the drill/screwgun for drilling holes now. They will last 2-3 years with extremely hard use. Batteries will last 2 years if fully charged/fully discharged regularly. I agree about the XRP. Haven't gone thru enough of the Lithium batteries to come to a conclusion on them yet. The lighter weight is nice, especially on the nail gun, and the charge does last longer, but haven't had them long enough to endorse or condemn.
If you don't use them often, it doesn't matter what brand, the battieries won't last. They need to be fully charged and fully discharged often.
If you're talking lighter duty, intermittent work, and don't need to drive a 3" screw, brand doesn't really matter much. Just buy whatever is on sale, and figure on replacing batteries every year.