Personally, I like both bands, but I'd have to go with Radiohead being the better.
I find Sonic Youth sometimes uses to much distortion in their songs. Also, they tend to be very long, which sometimes causes me to lose interest.
Radiohead, on the other hand, could make their songs as long as they please and I don't think I'd ever get bored.
Sonic Youth has good riffs.
Radiohead, I think, is the most innovative band out there.
Then again, I'm sure alot of people would say that about their favourite band.