I'd go for the cheaper one - if it goes bad, it's only 7 bucks, and pleco's grow fast anyways... Besides that, a 7-incher will not put as much strain on the tank's bioload as a much larger one (don't know what tank size and filtration you have, but it's always good to keep that in mind as well!)
Keep in mind that every fish in a piranha tank is a potential meals (even a 12" pleco), but usually they do fine. I have two pleco's (one at4" and one at 6") with my reds for well over a year now, and some succeeded in keeping pleco's with p's for a much longer time, but you'll just never know...