I don't own the V3i but I do have a friend that has one, that let me borrow his for several weeks to test with the MX Sport and some other machines. So this is coming from someone who really don't have hundreds or thousands of hours of V3i use under my belt. But I think what I will say, will be of help to you. I've had the MX Sport since about a month after they came out. Mine has the updates to it. Over the years I've also had the MXT and MXT Pro with various coils. I actually sold my last MXT Pro and coil cache I had for it, to keep the MX Sport. I personally just liked the MX Sport better than the MXT. But you are asking about the V3i so here is my take on it.
The V3i that I used, was particularly sensitive to EMI. A lot of the settings had to be throttled back to keep it somewhat stable. Not sure if this is common with them or just this unit I had; it was like that with the various coils I had for it. The MX Sport is actually not that bad with it. I have hunted under high tension lines with it, and just a freq shift enabled me to hunt without much loss in depth or stability.
While the V3i has more ID "features" for it...I didn't really find any of them to be any more beneficial or accurate for IDing targets, than the good ole VDI number system.
Setup on the MX Sport is much faster and easier...no standard and expert menu like on the V3i. I much prefer the layout of the Sport.
Here is the thing that got me. The V3 is a complex unit with a ton of adjustments. You can probably make it handle most situations you will encounter detecting IF you figure out how. However, if you don't have an explanation of what those settings control or how to use them...then they are just settings in a machine and you will be at the point of never feeling confident in the machine that you have it setup to its best ability. If you look at other top end machines that are more complex....they have some kind of after market book or video to shorten the learning curve and explain settings. So with those, a person pretty much can get a feel for what each setting does and how changing it, will interact with other settings. The V3i has nothing like that out there available for it. All I had was the manual, this forum/resource area, and YouTube. Some things I figured out on my own and some things I didn't dare mess with. Ground filters, band pass settings, etc etc....
I felt the V3 was a more powerful machine; or could be set up that way if you could ever find a emi free area that would allow you to take advantage of it. But I didn't find the V3i to be any more accurate in terms of ID, than the MX Sport was. In fact, I sort of saw the opposite. I will say, the 6x8 SEF coil on the V3i was a GREAT combo. Of the coils I had to try, it was my favorite coil combo for it. The 7" DD coil on the MX Sport is my favorite coil for it. The 950 and D2 coil were my least two favorite coils on the V3i.
It would take some getting used to, but I also felt the audio on the V3i was too overly digital sounding. That sounds silly saying it like that, but I like the audio on the MX Sport better. The recovery speed and audio duration of the MX Sport seem more pleasing to my ears vs the V3i...which to me, always seemed like it had too long of an audio response per individual target...and that there was a slight lag in audio from when the coil passed over the target til I heard the sound. I tried messing with the recovery speed audio of the machine and got this closer to my liking...but still not as good as the MX Sport.