In terms of NAND read/write speed, the NAND-X is better because it's faster, but the J-R Programmer offers more functionality.
The NAND-X is based on code by Tiros and it was never released to TX is limited to what they can do/offer with it, but the J-R Programmer was designed and built by them so features of their own can be added (take Sonus programming for instance - this isn't possible with the NAND-X because the ARM code isn't open source).