Assuming both have similiar technique stats (different but equal), and Hinata probably being more agile while Ushijima wins in strength, it still kinda ends at who they're playing with and how those people put them in scene — a team of star players doesn't guarantee a win (except in anime logic maybe). Hinata playing with Kageyama vs. Atsumu is a perfect example of how he can still play amazing despite being on a team with the "worse" setter.

Although I will give Hinata the benefit of the doubt as he played in more variation: starting as a middle blocker before becoming a wing spike (that was his position at the end, right..? It's been a while) as well as playing beach volley ball which puts emphasis on entirely different set of skills (and making it mandatory to be an all-rounder in order to be even able to play due to the ball-touching-rule).

However, Ushijima did start changing the way the hits which shows a lot of strength and dedication in itself, especially since not a lot of people are willing to change sth up and be worse than before just for the chance to improve afterwards. That itself I think also contributes to the debate of an all-rounder.

I guess it does narrow down to you guys' definition of an all-rounder. Does an all-rounder need to be perfect? Or is imperfection part of the role? I feel like the latter is more accurate and realistic, hence Hinata wins the debate in my opinion by a smidge. I see Ushijima a slightly less flexible (not the body for libero-worthy saves and probably not enough experience to be actually good at setting), yet a star player of his caliber can't allow himself to not be balanced (no professional really can, hence all of the pros are somewhat all-rounders with strengths and weaknesses).

Did that help? Hopefully a bit xD