I've had 'em all (including the IDX Pro), and based on what you are wanting, I'd strongly urge you to get the MXT All-Pro. It's a simple knob-based detector like your old IDX Pro. It's also a fast recovery detector with great analog-type audio that gives you lots of information, also similar to your IDX Pro. Additionally, the MXT Pro will give you the option of the 7 Tone ID, which is really the only reason for the M6's existence. The M6 was created back when White's only had the non-pro MXT without the 7 Tone ID and so the M6 brought Tone ID to the MXT-type detector line. The MXT Pro combines the best of the MXT with the advantages of the M6. If you have the All-Pro, you'll have the option of running the detector *just* like an M6, i.e. in Coin & Jewelry mode with the 7 Tone ID on, and even silent threshold if you like.
MXT All-Pro = MXT Pro, there is no difference whatsoever except for the little red sticker on the All-Pro.
Why not the V3i? It has a totally different personality. Try them both side-by-side and you'll see immediately what I'm talking about. It's got a very laggy all-digital way of operating that feels very disconnected from what's going on in the ground under your coil. The V3i is like a floaty 70's Cadillac - power everything, but very detached - in comparison the MXT is like a great handling sports car (with crank windows and no AC

).