SP101's are really solid guns, great for concealed carry and open carry alike. The 327 Fed cartridge also offer's better performance IMO. My first handgun was a Ruger SP101 in 357 mag.
The Beretta 81 is a fine piece, and I'd like one of those in my collection, but if only choosing one, I'd take the revolver every time.
It really depends on your intended use.
I should note that I've also had a 101 in 22lr. I got rid of that gun because I didn't care for the trigger, and the fit, finish, and overall QC were subpar. So as with any revolver, really check it out before opening your wallet.