I've got an OB-Xa on my bench for a tune-up, and my own OB-6 sitting here...
First, in the oscillators: the OB-6 has continuously variable waveshapes, the Xa are switchable.
There's a mixer on the OB-6 - no mixer on the Xa.
There's a sub-oscillator on the OB-6, no such thing on the Xa.
The Xa has 12 or 24dB/oct filters; the OB-6 is 12dB all the way
OB-6 has different filter types (BP and HP, in addition to LP), the Xa is just lowpass
OB-6 has velocity and aftertouch
OB-6 has phrase sequencer
Xa has some different performance options for the levers that the OB-6 doesn't have
Xa has second (mod) LFO that the OB-6 doesn't have
OB-6 has digitally clocked analog oscillators, Xa is all analog. Detune knob set to 40% emulates the Xa's nominal tuning slop just fine.
OB-6 has built-in effects (which are simple but appropriate), none in Xa
Panning is under the hood on the Xa. On the OB-6 there's a pan Spread amount on the front panel, although I haven't gone digging yet to see if there's individual voice pan. There's also a Mono software switch if you want to go old school.
Xa has split/layer, OB-6 does not
Sonically, the OB-6 is certainly capable of playing in the same ballpark as the Xa. In the first half hour in front of the unit I was able to mimic A1 "Jump" and A3 PWM strings very easily, just for shits 'n' giggles.
I've had multiple Xa's and 8's (and still have a 4-voice and an M-12), and for studio use perhaps it'd be nice to have an OB-Xa or 8 around, but the OB-6 more than covers that sound.
YMMV, etc. But that's what I think after a half-hour with the unit. Stay tuned...