I've had both in several configurations... still have a bunch of Paras. AFAIC, the steel is pretty much a wash. I like 'em both for an EDC folder. The reason I didn't keep any of the 551s is that they didn't carry as well for me as the Paras do. I like the flatter sides and G10 that Paras have better than the Noryl (FRN) and rounded scales of the 551. Both were light and easy to manipulate and BM's AXIS lock worked perfectly for me every time, just like Spyderco's comp lock. It just came down to personal preference based on ergos and aesthetics. In that kinda situation, I'm usually married to the bug.
"If there are no dogs in Heaven, then when I die I want to go where they went." - Will Rogers 1879-1935