I've used both the MXT and V3i in my gold hunts...in the gold fields of northwestern Arizona and in the Jamestown area of northern California. Here are my thoughts...
Given that gold is a poor conductor, it excites more readily in a higher VLF frequency. Operating at about 15kHz and 21kHz respectively, both machines are adequate for all but the smallest samples.
I think the MXT, even though it operates at a slightly lower frequency, is more suited than the V3i. It could be that I've found more gold with it too.

The MXT, is only slightly lighter in weight, but seems less bulky to operate...your results may vary.
The E-Tracs are popular in the gold fields of the desert southwest. I've seen some pretty nice stuff

come from the few gold prospectors that will share information.
I was fortunate to come across a Fisher Gold Bug II, new-in-the-box, from about twenty years ago that is my go to machine here in the desert. It operates at 71kHz. It's a real handful to get ground balanced and noisy as a truck full of crickets, but us old MXT'ers love lots of chatter.
Fisher dropped the 71kHz operating frequency a few years ago. I think the new models are around 19kHz. It's a very long story how I found mine...save that one for later.