Do not buy the bike that is too small and requires a handlebar extension. Does REI have that one in a larger size that fits? If not, then cross that one off your list. You don't want to be monkeying around with handlebar extensions to force a fit that doesn't exist.
Disk brakes are trendy, but rim brakes are fine. I actually run rim brakes on my go-to mountain bike.
Many will be so-so on the Novara because it is a second- or third-tier bike brand. You've got your top-tier brands like Trek, Kona, Giant, Specialized, and then Novara is just a store-brand. It's like the difference between Del Monte green beans and Spartan-brand green beans. (Which is to say that much of the difference lies in the marketing and in one's mind).
The really, really important thing here is to get a bike that fits nicely from the get-go. If you like the Novara, see whether one is available in a larger size. On the other hand the Kona Dew line has been around forever -- my son has one -- and has been successful for Kona. The Dew is available in six different frame sizes, so it should be easy enough to find a good fit.
If you like the Dew but want disk brakes, then bump up to the Dew Plus or the Dew Deluxe which have them.