I'm in the same situation - having both B. Roberts and Hill. I'm in a keeper league too, so at the end of the year I can keep Roberts for a 4th rd pick or A. Hill for a 23rd rd pick. This gives Hill extra value as keeping him is so cheap.
Anyway, if you need SB + R then keep B. Roberts
If you need HR + RBI keep A. Hill (although, most people project Hill to only finish w/ 22 or so HR, giving him 10 more HR over the next 4 months.)