Demo both but my experience (own 2019 Rebels but ride mates XR5/6s every now and again) is pretty much:
- XR6 higher harness pressure, rebel more balanced between front and back lines
- Rebel more explosive jump feeling than XR6
- XR6 jump sweet spot a little wider.
- XR6 possibly a little more stall resistant to poor bar trimming
- XR6 better if you want to also ride on a hydrofoil/surfboard (though still not brilliant - get the Nexus/Dice if you want more allround)
- XR6 easier access to the last 10% of top end. Needs a bit more technique and leg strength to get the max wind range from the rebel (just change down anyway as you'll go higher)
- Click bar is nice but the new Sensor 3 bar is a thing of beauty.
- XR6 needs to be packed up carefully to get the air out of the struts. 2 dump valves on the rebel so you can roll tip to tip if its really blowing.
- Velcro tabs on the XR6 excellent to keep the bridles tidy when packing up/setting up.
- XR6 canopy beautifully tight on the beach. Rebel flaps a little more on the beach. Both 'tight as the mate who never buys the drinks' once in the air though.
There's not a huge difference between them so you might almost pick based on which bar you like better

My personal take is that heavier people prefer the grunty feel of the XR6 more than the rebel. I personally wouldn't trade my Rebels for a set of XRs (despite it looking like the above says the XR6 is better) so you'll probably prefer one to the other back to back
Plenty of other good options though. Ozone Edge and Airush Lift look fun to ride too.