Hi,
I currently own three PI units - the Minelab GPX 4500, White's TDI, and Garrett Infinium.
In general the GPX is my nugget machine, the TDI I use for in-town detecting (tot lots, beaches) and the Infinium is my underwater unit.
If I were going meteroite hunting I'd probably go with my GPX-4500 and the Coiltek UFO coil (12" x 24" mono) not so much for depth but for ground coverage. The TDI would be my other choice as it is easily hip mounted which makes swinging the stock coil very nice for long hours. In theory I could use the UFO coil on the TDI but the UFO coil would be too heavy to use on the TDI hip mounted. The GPX harness and bungee make better sense running the big coils.
Truthfully, they all work. I think getting out and putting in lots of hours is more important than which particular unit I'm using. A person with the "best" unit who only hunts a few days a year is not going to do as well as that guy with the "not as good" unit who gets out constantly and really gets in lots of hours. So at some point just bite the bullet and go.
Steve Herschbach