I have both. I DD a RRS Supercharged and my wife drives an X5 4.8. I have spent quite a bit of seat time in both. The RRS is definitely more truck like, and while it does have all the nice creature comforts etc, it is not the same ride as the X5. X5 is more comfortable and surrounds you better. My wife likes the visibility better of the X5 with the large rear passenger windows and prefers the tighter steering of the X5 as do I. (has the sport package).
I personally like the seating position of the RRS better and do like the looks of it. You really cant go wrong with either. Just seems like every wife in the neighbourhood drives an X5. lol
Seating and cargo space is pretty similar. RRS looks much larger but really isnt - just taller.
Drive them both back to back if you can and make a choice. You really cant go wrong. I have had zero reliability issues with the RRS. (or the X5 for that matter)