NV3550 is a great transmission. If and when mine dies I'll be trying to find a replacement 3550. Mainly because the first gear on the 3550 is 4.01 and the AX15 is 3.83, losing crawl ratio is the wrong direction IMO.
The AX-15 is easier to source and find parts for. IMO the 3550 is a better trans but it was only made between 2000-2004 for the TJ and 2000-2001 for the XJ.
Since most people don't have the tools to work on the 3550 finding someone to rebuild it is quite difficult. On the other hand, the AX15 was around for 12 years so parts are easier to find, people can rebuild them, and remanufactured units are easier to come by.
Also, from what I gather the ax15 is a plug-and-play swap. (don't quote me on that)