I prefer Arrington. Either one of them would be a great pickup, though.
I think Arrington's size and abilities fit the position better, despite Julian Peterson playing the exact position for Saban in college. Julian Peterson's best fit, IMO, would be a blitz heavy 4-3 team where he could play backed up off of the LOS.
Strongside linebacker in the Dolphins base defense is lined up on the line of scrimmage, and has to constantly fight off blockers on rushing downs. I'd prefer a bigger, stronger player for that.
I also think Arrington is a better blitzing player. Peterson is no slouch in that department, and does have better coverage skills then Arrington, though. But Arrington is a bigger, meaner player who can deal with blockers every down.