I think they both suck, so I would just take whoever fills the bigger hole on your team.
With Peralta, I think you get a 4.xx FIP pitcher that, if you're lucky, Lucroy frames into a 4.00 ERA guy with a middling amount of K's, and below average (for fantasy) OBPA and K/BB.
You can get similar pitchers on the waiver wire throughout any given season.
Lorenzo Cain could go 10/20 with a 100 wRC+ (think .260/.320/.400), but those are mediocre numbers and CF is not thin in fantasy baseball right now.
Honestly, I think you can get more for Peralta than Cain, based on the fact that Peralta is young, seems promising, and SP can get pretty thin in deep leagues. I would continue to shop Peralta but look for something a bit better.