2nd Lens for 5D MKIII

Hello all, I have just bought the 5dMKII with 24-105 kit lens. My question is, for shooting portraits with off camera flash (strobist style), what should my second lens purchase be? I'm thinking a prime maybe 85mm or 135mm? Would 1.8 be too cheap matched to my sensor if I went 85mm? As I want this lens for portraits nice bokeh is one of my main wants. Budget is an important factor after being ripped off by the MKIII.  
Thanks everyone in advance!

--
MJK