The pronunciation is /ju/, so I would think that "you" is a closer rendition. The only reason "u" works is because we happen to pronounce the name of that letter exactly the same: /ju/. So it's being used as a non-traditional digraph.

It's the same pattern as using "8" in examples like "g8 b8 m8 I r8 it 8/8". Here "8" is serving as a non-traditional trigraph /eɪt/.