I don't think you'll find many people who've used both.
We (My team, plus the four others run through BadgerBOTS) use Tetrix, and it's... alright. It has a lot of flaws, but I don't know if the Matrix kit is any better; I've never used it.
Tetrix has the advantage of being far more popular (at least in the USA), and so you'll have a much easier time getting help from other teams, borrowing spare parts, etc.
Honestly though, we try to stay away from the kit, because while it's nice for quickly slapping things together, Tetrix rails and parts are much more expensive than custom-made pieces, and generally weaker. But that strategy is dependent on you having access to at least a basic machine shop (Which pretty much every FRC team does already). For instance,
here's part of our drivetrain from this year. There's some tetrix parts up above (Frame, motors, sprockets, standoffs, axles), but the majority of the structure is either custom machined (There are some custom axles and spacers inside there that you can't see too), or off-the-shelf (Andymark wheels, sprockets and bearings, 80/20, etc).
Overall, I find the Tetrix kit to be pretty frustrating, but that could be due to the fact that I'm much more used to making and designing things from scratch. And again, for all I know, the Matrix system might be just as frustrating.
Those little Tetrix D-shafts are pretty much junk for a lot of applications, you can't really transmit force through them, because the set screws will just dig in and the hubs will spin on the shaft.
Tetrix motors are also notoriously prone to burning out if you stall them for more than a second or two. And if you attached an encoder to that motor, you're out of luck, because the encoder can't be removed without damage. Again, I don't really know if Matrix is better in this regard.