Hey there David,
Great Choice on the ATPRO!
The Stock Coil on the ATPRO is more than Sufficient for most ANY terrain and Mineralization. I would only suggest the 5x8" coil for 'Trashy Areas' as it has Excellent separation capabilities and still gets quite decent depth.
You can find Meteorites just as easy as a bottle cap, but remember of course that you'll need to be in All Metal Mode and you'll also be picking up 'space rocks' to a certain depth. There would be NO difference between a V3i and the ATP if detecting a 5 or 10lb Meteorite at 18" or more and I would say that the ATP might even have the advantage (it picks up large targets VERY deep, especially when messing with GB above Neutral!)
Having said that, you're better off with a modified '2-Box Detector' for searching Meteorites. The ATPRO will certainly find them, but your 'lack of depth' would likely be an issue for most worthwhile Meteorites. The 2-Box system will allow you to hunt VERY deep for Meteorites (large ones) that would be worth some decent cash. There are modifications that can be made to some detectors that will allow you to actually 'drag' a coil behind you (made from PVC or similar) and that can really make things interesting in nice 'wide-open' spaces. There are 'other Detectors' that you could use (NOKTA, OKM, etc... but that's a whole 'other' piece of machinery..