Some not entirely scientific benchmark comparisons across a variety of reviews, largely focussing on anandtech's bench. I think I had to cross derive between reviews looking at the A4-5000 and the A8-4555M, which are similarly clocked quad-thread APUs (the Trinity ULV has the advantage of boost for single threaded loads, of course). In well-threaded loads the performance was eerily similar
I'm not sure if Mantle is Windows only or if a linux flavour will be released eventually, but it's probably only going to directly relevant to AAA titles anyway, so I guess for a SteamOS/Linux box it makes sense to look at nvidia. Which makes it a real shame that their lower end cards just aren't price competitive with the AMD cards on Windows: we know we'll be losing absolute performance just because AMD haven't got their Linux drivers in order... *sigh*