I don't know about the Garrett GTI but I own and use a V3i and I love it. It is not an easy detector to just pickup out of the box and use. You will have to learn how to setup the programs (just like the Minelab). A friend of mine owns an E-Trac and it is a great machine. We hunt together most weekends and the only major difference in the machine's, that I have noticed, is the wireless headsets on the V3i. Working the program menus on the V3i looks a little daunting but you get use to it pretty quick. In addition, White's has lots of V3i programs on their forum that are freely available. These are not the factory programs but programs other V3i owners are using in the field with great success.
Another reason I own a White's Spectra series detector is that it is upgradeable. I originally bought a V3 and when White's came out with the new V3i all I had to do was send it in with $25.00 (for shipping) to have it upgraded to a V3i. I did not have to buy a new detector.
However, the right detector for you is a matter of taste. Try both the Garrett, Minelab, and the V3i before you buy them. They are all excellent detectors. Good luck.
