I love Nirvana too, don't get me wrong. I own really rare first pressings etc., etc. However, Radiohead is comprised of incredible musicians, brainiacs who can write lyrics, compose tunes and create an entire genre of music. Nirvana broke away from the corporate whoring that was glam rock and 80's pop but seriously, what they did, could have been done by someone else (in fact it was...by Neil Young 2 decades earlier).
I listen to Radiohead more, and I place them in my top 3 bands of all time usually in the second spot but depending on the day they take top spot. So perhaps I'm biased, but I have Nirvana in my top 10...so I'm not horrible opposed to Nirvana either. I just think that of the two, Radiohead will clearly be the one to write a PhD dissertation on. Nirvana...not so much.