Well, there are pros and cons to each. The glass Lixit are dishwasher safe as long as the nozzle goes in the top rack. That way you know they're sterilized (by the heat). The glass also makes the only chewable part the plastic cap/spout. But, ime, the Lixit do drip (there's actually a drop hanging off of one of my Lixit bottles right now), and I've dropped and broken one while washing it.
I haven't used a Nivek yet, but they're very similar to the Edstrom Water Buddy, which I have. That's the valve bottle my guys can't get enough of, so clearly there's something about it that's easier or more fun or something. But, it's made of clear PVC plastic; that usually means no dishwasher (or top rack only for some) and chewable. With those, it's usually recommended that you use a guard.
Maybe try two and two, so you know at least one will be safe/working at all times? You can always resell on here if you see that they're just not interested in one bottle at all.