Here are my thoughts after trying various radios over the past two weeks. I live in Stafford, VA - midway between Richmond and DC. I'm using Sprint, and I run push mail, auto weather, and Palringo, so my data is pretty much always on.
The first radio I tried was the Telus. I really liked it; better signal strength over stock, and much better battery life. However, it has an annoying glitch that I cannot live with. Sometimes, after switching to 1X mode, the phone will not switch back to EVDO when I go back into an EVDO area. I noticed this when driving home from the store; had EVDO all the way except for a quick 1X-only zone just before getting to my house, but within my house, I do have an EVDO signal. In fact, I use a Sprint data modem as my primary Internet connection. Anyway, the phone stayed in 1X and wouldn't switch over; according to FieldTrial, EVDO signal was ~-80dBm, but the conenction was stuck in, "0 - Inactive." I had to soft reset to get EVDO back. It only does this with the Telus radio, and I really can't be soft resetting my phone every time I drive through a 1X-only zone.
The newest Sprint and Alltel radios were almost as good as the Telus with regards to signal strength and battery life. However, they did something weird with the EVDO at my house; signal strength was good (~-80dBm), but when downloading a file, the connection would flicker on and off every second or so. "Requested data rate" would flash between 0 and 2000+kbps, thus bringing the average speed down to ~500kbps. All other radios settled for a constant connection at ~500kbps, which I suspect is better than an unstable connection at 2000kbps.
Overall, however, the Sprint and Alltel radios were a good experience. GPS was rather slow, though, and I couldn't get it to work at all on the Alltel (yes, tried deleting the SUPL AGPS key).
The Verizon radio I've found to be much more aggressive than the others. It will do
anything to maintain a signal, even if that means catching your pants on fire. For example, I go to church on Sunday, where I'm always roaming and have a very weak signal. Inside the acoustically-insulated sanctuary, other radios would just give up and disconnect. Once, however, I went in with the Verizon radio, and sure enough, it maintained a 1X connection - but drained the stock battery from 100% to 20% in a 90 minute service. That is not an exaggeration; the phone got hotter than it does when I use WMWifiRouter. I don't know if the phone is boosting its output power, running some heavy amplification circuitry, or both, but apparently gobs of current can somehow get you a usable signal.
For additional confirmation, I noticed a dead spot in a rather remote pet store where I couldn't get any signal at all with the Sprint and Alltel radios (never tried the Telus). I took the phone to the same spot with the Verizon radio later; not only did it have a signal, but it had an EVDO signal. FieldTrial revealed that EVDO signal was -100dBm, but sure enough, it was still connected. Not thinking this was possible, I did a mobile speed test, and got 150kbps - not fast for EVDO, but certainly better than nothing, which is what I'd expect (and got, from the other radios) with such a poor signal. Had I left the phone in this spot for a while, I'm sure it would have killed the battery, but the Verizon radio did indeed seem to grab a signal where no other radio could.
As I mentioned before, I run Palringo constantly. With every radio except Verizon, I have to check it every 15 minutes, as at least one IM service would have disconnected for some reason. With Verizon, I stay connected to five services all day long. Even if I do manage to find a spot where even Verizon cannot maintain a signal, Palringo is able to properly suspend and resume the connection without my intervention; this doesn't happen with any other radio. I'm not really sure why. Something always screws up (I get #777 error, or Palringo thinks it has a connection when it doesn't, or Palringo never resumes the connection when the signal comes back) and I have to manually reconnect.
Therefore, this is my advice so far: if you spend most of your time in good coverage and only drive through dead spots, go with the Verizon radio. It will give your phone a power boost when necessary to avoid dropping calls on the road. However, if you spend extended periods of time in low-signal situations, you probably won't be able to stand the battery life of the Verizon radio, so go with the newest Sprint or Alltel if GPS works for you.
Since I have a 3000mAh battery, Palringo, and decent coverage at home and work, I'm sticking with the Verizon radio. I'll just have to remember to turn the phone off at church, which I guess I really should be doing anyway.
^NOTE: Take everything I said above with a grain of salt. These results are very area-specific, and unless you have the same phone and same commute as I do, the only way to find what radio is best for you is to try them yourself. I'm just reporting my results in hopes of providing a starting point.