Sorry OrPe, I don't have any saddle time on the Orbea Onix. But I have had considerable time on the CAAD10. For me, I would go with the CAAD10 hands down. It is a race frame, yet is comfortable. Also does a splendid job of getting the power to the ground.
Not to throw a monkey wrench into things, but if he is looking at second hand options and the CAAD10 frame geometry suits him, I might recommend that he takes a look at the SystemSix Hi-Mod as another valid option. I think the SystemSix is a much better frame than the CAAD10 overall. Can find them on eBay or CL for decent deals. If he isn't against it, he could piece together a decent build himself. I saw a 56cm (non-HM) go for $275 the other day on eBay. Frame was in great shape. Just a thought. But in summary, I don't think he can go wrong with the CAAD10 at all.