Since the games are running off cloud servers I would expect the graphics quality to be the same or at least very similar. I would think the image quality would depend more on your internet speed than the hardware you are running it on. That was a common complaint when I read about onlive game streaming too though. Supposedly the image is overcompressed and the visuals suffer.
I use a wired 20+ mbps connection, so it probably can't get any better than that--though I should probably test it again to see if they've made any improvements over the course of the beta period. After tomorrow, I can do a direct comparison of Puppeteer, which becomes free for Plus members.