I think both Virtuix and Omni have their good and bad points:
Omni's biggest seller is that you can get really going in terms of speed! I've never seen any Virtuix demo that matches it. If you're into fitness, this means that your aerobics will be really good here. Also, the bowl shape is probably more natural feeling in the long run.
The Virtuix doesn't give you the sheer speed, but it does other things that I don't think the Omni does that gives it an advantage.
Firstly, it has a decoupling system now, so you can look one way while still moving in a different direction. It also allows you to back up. Omni decided not to do this because once you're immersed in a game, it will be more natural to just turn around and run. Great logic, unless you're playing a game like Skyrim. Mage characters have to be able to move backwards in order to maintain the space between themselves and the enemy whilst executing their attacks.
It also allows you to sit and crouch. Now, if you do a lot of crouching, that's great for strength and endurance, actually. You don't get the aerobics that you get from the Omni, and it's probably not that useful if you don't crouch for long periods of time, but if you're the type who does that kind thing, then Virtuix is really going to shine.
I guess it depends on what kinds of games you're going to be playing, since each machine sports features that cater to certain types of players.
As for me, I wish I could combine the two, actually.