I haven't had the chance to shoot an SP-1 but my old man (left-handed) claims that the rounds bounced off his cheek regularly. Someone else can give you a little more description but that's my .02
Quite honestly, aside from adding an SP-1 to a collection for history's sake, I would avoid it if it was a choice between ONLY an SP-1 or the R6551.
Please take this with a grain of salt. I'm only 19 and havent gotten the funds for a pre-ban yet. I've done a lot of shopping and I must say that I wouldn't give up a R6551 for an SP-1 due to its limitations/age.