I'll take Harvin and the higher floor. LaFell may have a higher ceiling, but not a lot higher. We've seen journeymen receivers have periodic success with the Pats before; it never lasts.
I'd go with LaFell, but limit my expectations, because like wilwhite stated, it never lasts. As for Harvin, I just don't see anyone doing well with the Jets offense. Right now they are giving him lots of looks, that will change IMO and in the meantime don't expect TDs from him. I think LaFell will at least give you some TDs off and on.