I attended a kick-off event for the G01 X3. I would recommend you actually look at it. When I did my write-up of the event, I compared the actual specifications of the new G01 X3 with the current F15 X5 and confirmed that the only real size difference is in the rear cargo area and a bit in width, but the seating areas are nearly identical.
I agree that available interiors of the X5 are nicer than the X3, but base level interiors are comparable.
Here is the link to the thread of the G01 X3 Kick-off event:
http://x3.xbimmers.com/forums/showthread.php?t=1432572
Post #44 I provide my overall impression of the new X3.