It all depends on your needs I guess. For stumbling, you just need to know where you are at a given moment.
The Garmins seem to have less interface problems than the magellans, but as long as it can talk NMEA, it does'nt matter.
The question comes down to cost and extra features. As BW said, the etrex basic will do you just fine for wardriving, but the legend is nice with the onscreen maps (useful for backtracking your way out of suburbia hell and getting back home). I personally use the Legend. Others have more memory and altimiters, barometers etc.
For the low end of the scale, I ordered one of the $15 Motorola cards from this thread http://forums.netstumbler.com/showthread.php?s=&threadid=4188
for some prototyping, and it should work no problem. 8 channel, NMEA output. What more do you need?
The make should'nt matter. The only thing to look for are;
1. 8 or 12 channel. Both work, but the 12 is better at keeping a lock. Those tree lined residential streets are a pain.
2. NMEA output. You could also use the Garmin text output, but other apps might not like it.
3. Serial interface. Make sure it has the proper interface that will work with NS. USB gets iffy.
Hope that's a decent analysis.