My brother in law bought the RX8 a few months ago and I took the liberty of test-driving it before I bought the XF.
My two cents:
I'm more comfortable in the XF.
Was not thrilled with the styling of the RX8 from some angles.
Loved the RX8 navigation.
I felt like all the RX power was at very high revs, if you like to drive that way then that's not a big deal.
Rather have the traditional MBZ drive train than the rotary engine.
Just FYI and I don't know if it is typical but his RX8 uses oil. The dealer says it's normal but I don't know about that.

Don