I was in the same dilemma and finally went with the Colt/Walther. It was very tempting to go with the CMMG or other .22 upper then swap on my lower but in the end I liked the idea of two rifles. So for me I had to choose between the Colt and S&W. I have to say I like that the .22lr rollmark says Colt and M4, plus it has the heft of the M4. I've had no reliability problems with it since I quit using Remington Golden Bullet so I'm happy.
Lots of good choices though.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek