Detection range, and real time filtering.  The best ones out there are from Escort, and Valentine.  I have the Escort 9500i, which is awesome at detection and at filtering false alerts.  The Valentine V1 has a tiny bit better range, but will go off constantly if you're in an area with a lot of door openers.  You're talking $400+ for these though.

I have 2 $100 Cobras, and one $200 one, and they're excellent.  I've never had a problem with them detecting anything.

ive used both whistler and 3 different cobra models.  I liked em all.  Sometimes the only differences are 360 degrees vs. just straight ahead, distance range and false alerts.  most cobra's are decent but obviously the more money u spend, the better it will be.  check online at amazon, ecost and other places for cheap deals

my cheapo cobra will pick up anything around me. just doesn't let me know what direction it comes from

i keep it on highway. keeps me paranoid. every time i go by an automatic door it goes off. annoying, but keeps me off the gas