Here's my take since I have both
The U5 has six different preset EQ shapes. I would say half of them are very useful in a live setting, but all would/could be useful in the studio.
I would
disagree that it is sterile sounding, but it is definitely clean. The #1 and #2 presets are big and warm, with some decent mid scoop going on. When I used the U5 live (with an SWR 750x) I mainly used the #4 preset. It really enhanced the midrangey sound of my Thumb bolt on.
The Sansamp RBI is like a SVT in a lightweight box, minus the adjustable midrange. This thing does big and warm, clean and snappy, and it gets fairly dirty. 3 bands of EQ, presence and a blend make it very versatile. Great for pickstyle playing.
Most bassists would get more out of the RBI than the U5, IMO. It might take a little more experimentation incorporating it into your rig (FX loop vs straight to amp input), but it rocks once you've got it!