FWIW. "it is said" that JTDX is based on the old KSVAD decoder, while the latest WSJT-X releases v1.7 uses some sort of newer/better decoder. It "is also said" on some less-than-scientific comparisons that JTDX decodes fast, but generally misses some signals that WSJT-X v1.7 rc3 catches.
Just "by eye" (which may or may not be true) WSJTX v1.7 rc3 decodes more signals, and closer together signals. It's now not uncommon to decode 25 or more signals on a busy band. It looks like it's about the same speed to me, but decodes quite a few more JT65's than v1.6. It's also fairly common to see 2 or 3 signals decoding on the same Hz.
I also like the meteor scatter mode MSK144 - very cool.
If anyone has a link to a properly done head-to-head test, I'd be interested.