I'd lean towards the Milwaukee if these are the 2 options. If you threw the Flexvolt in there instead of the 20v, then I'd probably lean DeWalt. Even if you don't think you need the extra capacity now, you most likely will at some point down the line, and I'd rather have that capacity now and not need it than not have it when I need it in the future. That, plus the fact that you're already locked in with the red, and it's nice having a bunch of tools with the same battery pack so you can mix and match when need be.
I guess if you're going to be moving around a TON, then the lighter DeWalt could make more sense. But even then, the Milwaukee isn't THAT much heavier. It won't be as convenient, but as long as it's still manageable without putting too much stress on or hurting yourself, the extra capacity wins out for me.