Waiting for thawed ground to try out the V3i, I just got it two months ago. So I cannot tell you from experience.
I was given a disease from my father, I have challenges with parting with my money. LOL. my knowledge of these two machines is from about 7-8 months of reading, watching videos and talking to dealers.
Based on what I've learned, if you can afford a multi frequency machine, get it. You will get many opinions here and mine is just one of them. I started with a ace 350, then the ATPro and now the V3i. While I loved my ATPro and enjoyed using and will not sell it. The technology is "toyish" compared to the v3i. It's about like comparing a slider phone to a smart phone.
Again not to offend anyone just my opinion.