They are very much like the debate regarding the 109F-4 vs 109G-2. Ultimately, one has more speed and the other has more maneuverability.
All things considered, I prefer the -9U, it has more range and is faster. Range being the bigger factor. They handle very similar with a slight edge going to the -3 in the in close knife fights.
Which Yak model is "better" for a said task should be the discussion, not "which plane is best".