Not heard the Rega Brio-R, but it seems to have a enviable reputation as a all-rounder. I have heard, on the other hand, Exposure 2010S (both amp and CDP) connected to MA RS6, and to my ears it's the best sub-£1000 amp I've heard for rockers.
Really the best advice would be to audition both and see which you prefer.
I would also consider your sources. Both Rega and Exposure aren't gifted with a shed-load of inputs. If you have a TT and a whole raft of analogue and digital components, you're probably best looking elsewhere.
Yeah! Had the chance to audition the brio r briefly....a very good amp. I have yet to audition the exposure 2010 this week. They are priced relatively close. Choosing between the two is a hard task that I have to undergo.