They use the same chip - NTAG216 - but the primary difference is performance and installation method. This thread talks about people's experience with the flexNT.
However, while the flexNT outperforms the xNT, the installation is currently orders of magnitude more involved. The xNT is simply injected.. a professional body piercer can easily do this. The flexNT requires a scalpel, a dermal elevator, and sutures to stitch the wound closed. This is beyond what most body piercing shops can do for you... but not all. We do have a partner map that does categorize professionals a bit so you know who you can talk to.