i'm using gm02ew on my vehicle, it's small, and has pretty many functions that i need so far.
And yes, cellphone nowadays can provide a tracking services, but it's too BASIC. they only tell your current location, that's it.
But vehicle GPS is a total new thing, i can check the real location, sending the commands from my phone, website and even via SMS ( although i don't use sms too much :p), set the vehicle in a distance area if someone stole it, ACC and area alerts will both notify me, then I just need to call 911. and most importantly, I can playback the track that I have traveled(GENIUS)!
So, It's very helpful when my vehicle lend to someone else, eased my worries and concern.
Hope these words that I wrote so hard can help you to find the best GPS for you !