UA's Y+ offers more room than AA's MRTC.
I suspect your upgrade chances are better on UA than on CO -- since CO offers unlimited free domestic upgrades, everyone is signed up for 'em so silvers have a tough time. On UA they aren't free (you earn 2000 miles of ugprades for every 10000 you fly) so not everyone requests an upgrade on every flight.
I'd stay away from CO b/c of their Economy-minus. Likewise, I'd run from DL if I had to sit in the back (and since I would be buying mostly L and U fares...).
As a Silver, coach service is important. I think the only choice is AA and UA.
Ultimately, the answer rests on
your travel profile
what awards are important to you
etc etc etc
It's an individual thing.