Some responses based on my experience with my Touch Pro 2 on Sprint.
roaming on Verizon ? no problem, WM makes it easy to force roaming if I want.
Sprint Nav ? excellent nav, I frequently travel with someone who uses Verizon and both navigate equally well, and often better than her stand alone navigator.
Google maps ? with WM, I have Google maps including street view, Bing maps, and Sprint nav. It's nice to have backups.
Sprint tv ? pretty useful actually, and includes overlooked Sprint radio which is actually very cool for listening to local radio from across country.
Nascar and NFL apps ? very cool for fans, Nascar one is great for a fan like me. live in car audio is great.
3g coverage ? not mentioned much but Sprint has much better nationwide 3g coverage than ATT or Tmobile, only provider that's close to Verizon.
rollover minutes ? who needs them when all mobile to mobile is free ?
Verizon is expensive but worth it.
Sprint is best deal if they stay in business.
ATT is an evil,dishonest entity in the same vein as Exxon or Comcast.
T-Mobile, I know nothing about.