I watch films on the 11" and they look fantastic.. both screens are HD quality.. stepping up to retina will get you slightly more pixels only when watching a 1080p movie.
Watch them all the time on my 11". It's the perfect size for an aircraft tray. The 13" is a little too big to watch while flying . . . unless you're in business class.
I do but have to say that about the only thing I preferred on my retina 13MBP's screen was watching HD video. It's OK on my 13MBA but not a patch on rMBP. For everything else I prefer the MBA.