It's been awhile since I've watched One Punch Man, but I think even if I rewatched the first season, I'd still prefer Mob Psycho 100. OPM has some really great action scenes and comedy, but I felt like they played tone a bit too straight after the first few episodes. The whole conflict with the different superheroes and villains just came off a bit too serious. I think part of the issue is that I thought that the series would be more comedy based with some action based on the first couple of episodes, but it turned out the opposite.
It is still a really fun series. The comedy is great, the action scenes are pretty awesome and I still hope that Toonami picks up season two whenever possible, but I do think that they played things a bit too straight.
Mob Psycho 100 definitely has better characters. Mob is a more interesting lead than Saitama, or at least a more relatable one to me, Mob and Reigen have a nice dynamic and most of the supporting characters get a good chance to shine or something to make them stand out. I didn't think Mob's brother would be anything more than a background character, but Ritsu going mad with power and exploring his relationship with Mob was great. Mob and Saitama are both overpowered leads and that works for both of them, but I do like that we saw Mob struggle with some opponents, as well as with his morals in regards to using his powers against people.
The more serious tone also kind of works more with this series since I didn't expect to be mostly comedy like I did with OPM. It still has comedy and it can be funny, but it felt like they were playing things mostly straight from the start, so dealing with the Claw organization didn't feel strange or jarring compared to the first few episodes. It felt more like a natural progression. Oddly enough, I was so sure that Mob Psycho 100 would be more episodic based on the first couple of episodes. Both series are pretty good and I hope that Toonami will air their continuations, but I think Mob Psycho 100 leaves a stronger impression on me than OPM did.