I've got a FlexDF, and I already love the design over the xNT. It's SO much easier to get a read off of it from my phone. I haven't been able to sit down and tap into the shenanigans it's capable of since I've been on the road all summer, but if given the choice of another xNT or a flex I'd spring for the flex.
HOWEVER the installation process was much more difficult! With the xNT it was just a quick poke and I'm done. The flex install isn't quite as simple; watching the video of my own install makes me queasy! You better make sure you either know what you're doing or have a legit person taking care of you, because as far as I know there's no easy way to inject the flex chips quite yet.
Good luck no matter what you choose. (Who says you even have to? Get both.)
Either way, welcome to the club!