I've now played just about equal of the FIFA demo and the retail release of PES, and unless the retail version of FIFA is much improved, at this point I could for the first time since FIFA 08 see myself playing PES this year. While PES is far from perfect, it's just way better at executing its gameplay than FIFA. (Again, I could totally change this opinion when FIFA releases.)
Both games have their good and bad, but if I were to end up choosing FIFA, it's likely because of three main reasons: control, error, and variety.
A huge advantage that FIFA has over PES for me is that so much control (and thus freedom) is given to the player, and I think this is at the core for why FIFA can feel like a more satisfying gaming experience, especially with scoring but also with passing. FIFA just puts more options in your own hands, while with PES it's highly assisted.
Examples of this include how in FIFA we control whether to shield or not, or whether to switch into a jockey stance, and FIFA offers more shooting options. The best example though is all the different pass types you have at your disposal in FIFA. Just look at the new dink pass options they added this year, which last year were purely assisted and contextual (as they are in PES). It's actually one of the few things I really enjoyed in the FIFA demo - succeeding at a dink pass or through ball feels great. Say what you will about it's execution in FIFA, but giving more agency to players over how they play the game is a huge deal in this age of gaming, and it leads to a more satisfying and rewarding experience.
Another big piece of this are the actual control schemes in each game. For me, I never struggle to perform an action in FIFA because of the controls, while PES in my opinion has one of the worst control schemes in gaming. It's incredibly frustrating in PES, especially considering how much more difficult attacking play is than in FIFA.
Error is a big one for me, and might be what prevents me from sticking with PES. As as been said many times, there's just not enough error in PES. But there's two other bits to this that are really important. One is that, while there certainly is error on shots and passes, there's very little error when it comes to the CPU decision making - unlike us, they almost always pick the right pass, the right touch, the right skill move, etc. Even more frustrating for me is there's very little error in CPU marking and tackling. @chrisdavies posted a video about FIFA's AI being psychic but IMO it's actually way worse in PES - they can feel magnetized to you and the ball, regardless of skill moves or feints or momentum. It makes it feel like I'm playing against robots.
The other bit when it comes to error is that PES is really bad at capitalizing on error. For example, there's plenty of times I'll get myself in the right spot for an interception or I'll create a 50-50 situation, but either my players don't recognize the opportunity quick enough, I'm locked into a movement pattern that screws me, or maybe the ball just bounces off of me.
Last is just the amount of variety in FIFA, and the feeling that anything can happen. IMO this is very much connected to the first two. Error is obvious - the more error and bad decision making, the more midfield battles, the more goal mouth scrambles, the more chaotic 50-50 situations. Now I very much believe that FIFA's variety is a double-edged sword, but still, I can't escape the feeling in most PES matches that I'm playing a straight-jacketed version of football, that any variety or sense of randomness is an illusion by the invisible puppeteer hands of the game deciding what can and cannot happen. PES is the actor that never breaks character but is limited with what roles it can play.
Last year at this time I thought PES might be the game for me, but then I hit a wall - especially with the AI - and ended up playing FIFA. That could happen again. At the moment I'm really enjoying the football in PES, but I'm at the point now where I need to move up a difficulty level and so maybe my experience and opinion will change. (Also, I still can't shake the utter disappointment with PES in pretty much everything non-gameplay related.)
I'm still holding out hope that FIFA 20 is ultimately much better than the demo would suggest, but at this time early impressions are that there's no competition in gameplay quality. The trick for me would be whether the three above factors don't drive me back to FIFA.