I just bought a new framing nailer and did tons of research, and one thing I'll warn is do your homework on the most common style of nail in your area. For some reason framing nails come in 21 degree and 30 degree configurations (and coiled), and it seems very regional as to what is more common around you.
I was about to pull the trigger on the Milwaukee when I found that they only make a 21 degree gun. Unfortunately in my area there are no 21 degree nails to be found, so I would have been signing myself up to buy nails online for forever.
Many companies make both types, so you have to be sure you know what you're getting when you buy (and what your local stores carry).
Between those choices, assuming 21 degree is good, I'd definitely go Milwaukee. By my research it was second to none. Although I have no experience with Senco, most of the info I found on them was that their nailer is just a little more out dated, heavier, fewer features, etc. Although I'm sure it would still be reliable.
Not one of your options but I ended up going with a Bostitch. Got a very good deal on Amazon, and another quality name in the industry. So far I've used it a couple weekends and I couldn't be happier. Zero jams, zero misfires, no issues at all. Tons of power, grips well for toe nailing. Very happy with the purchase. And you can get their gun in 21 degree or 30 degree as well.