Anyone that can play both well has my respect.
try the Hammerkalvier Sonata! It ends with a virtually unplayable fugue. I don't think he had actual performance in mind.
Ok, now I'll have to dig that up.
The Hammerkalvier Sonata is one incredible piece of piano music! Music like that makes me marvel at what kind of mind could create something of that order.
Look for Piano Sonata no. 29 in Bb, Opus 106, 'The Hammerklavier.'
Go for Sonatas 26 through 32. They're all rather interesting.