I was in your shoes about a week ago. I had Nextel for over 2 years and absolutely loved them. The call quality was FANTASTIC and my i870/i880/i580 never dropped a call.
After playing around with the 8350i and the Sprint 8330, I actually liked the Nextel version better. It seemed to feel better, it looked better, and the keyboard seemed "softer" than its Sprint counterpart (which I liked). However, iDEN data speeds are BRUTAL compared to Sprint's 3G capability. This is why the Nextel device doesn't support "streaming" like Sprint.
For me it came down to data speeds. If I was going to upgrade to a new Blackberry, I wanted 3G speeds that would allow me to utilize everything it had to offer. That is why I ended up going with the Sprint 8330.
Also, because I couldn't bring myself to totally ditch Nextel, I still have one of my old handsets activated on a Boost prepaid "Pay As You Go" SIM. All you have to do to keep the account active is add $10 every 3 months. It's the best of both worlds, Sprint 3G 8330 and a Nextel i580
Good luck in your decision, but if it was me I'd switch to Sprint.