Honestly, I've never been a massive Junpei or Yosuke fan but I do agree with Fynn and WK that Junpei is the more compelling character even if, superficially, Yosuke is more comedic and built for laughs.

They're both flawed in ways I don't appreciate, but I approve of Junpei's character development over the course of the game. Yosuke - and to be honest the majority of the cast - are more static and it is harder to detect moments of character development because it is so front-loaded.

In Persona 3, everyone already has their persona. You don't need to go through a dungeon to obtain them. Because of this, their arc bleeds through into the main story until they all get their ultimate persona at a critical moment.

Whereas, in Persona 4, each character introduction is preceded by a character development dungeon, which basically shows you a clear before and after image. Take Kanji. Before his character development dungeon, you see him as a brash delinquinent. Post dungeon, you see it's actually his problems with living up to some masculine ideal (and maybe sexuality, but that's a different argument), and how he works past it. Same goes for the other characters. Their character development post-dungeon is much more static because they've gone through their major character development simply in recruiting them, as opposed to the P3 method of interweaving it as part of the main plot.

In the P3 method, Party Social Links act almost like Gaidens or side stories that aren't integral to the plot but serve to facilitate it. And you get a reward out of them regardless (getting the top level persona of that arcana).

I will say I quite liked P3P's method, of having S.Links and main plot factor into development. But it really only works because your party members already have their persona. They join you straight away. The nature of Persona 4's writing is such that everyone has to go through a dungeon to claim their initial persona, and the plot dictates that they get their persona after some personal growth. Problem is, most of their growth happens when you first meet them (S.Links tend to reaffirm whatever message it's saying to gain their ultimate), so they feel static in the main plot because their primary problem is resolved.

Honestly, my main problem is that he doesn't really seem to grow out of his juvenile behaviour and thoughts. I don't mind Yosuke being homophobic initially, but they really needed to follow that up later on in the game and they just didn't. I agree with Fynn about it being interesting the idea of Yosuke being a gay option, and it is a shame they scrapped that idea because it would've cast his homophobia into a whole new light.

Junpei, for all his initial flaws, in my opinion he does go through some significant development and so for me is the more interesting character. Yosuke just seems a little too much like a cipher for 'best bro', and to be honest I kinda wanted that role to be given to Chie. Chie's cool.