Would not want any of those two. Fabián Ruiz is a superior player to Partey by some distance IMO. Havertz has a higher celling than Van De Beek as well.
The way Arthur has been performing and the type of player he is, there is no way that he should be sharing playing time with a player as good as Partey. A Seydou Keita type of substitute would be much better.
I believe that FDJ's best position in the midfield is that of Busquets. If we assume that FDJ will replace Busquets from next season on, we just have to partner Arthur with another midfielder. That midfielder should IMO be a more offensive and dynamic minded midfielder. Fabián would be ideal. Havertz as well if a more offensive midfielder is the goal.
Let us not forget Puig either who I hope will be given a chance from next season.
Exactly how I feel. NO to the likes of Thomas, or Van De Beek (even though I like VDB, but) Fabian and Havertz are both the real deal for me and are my top choices for that RCM role alongside Arthur and De Jong a little bit behind them, maybe even as the 3rd CM (in the middle) not pure CDM in front of the both CBs like Busquets, as I feel De Jong could play a lot more advanced and adventurous than Busquets, but still behind both CMs, not in front of them as a CAM, as I don't like the idea of us playing 4-2-3-1, cause at Barca LM/RM is basically LW/RW so 4-2-3-1 is basically 4-2-4, which takes numbers out of the midfield, which I feel like should always be our strongest part of the team.
I actually did this in FIFA just last night, as I started my Barca CM. I'm playing like I would want Barca act in real life, but still keep it at least somewhat realistic, so I don't go out and sell our whole main core in the first season and replace them with all the best young players. There are some exceptions, players I actually think would be realistic, or players I enjoy using too much not to buy them (like Mbappe in a few years), but I still don't go out and buy a whole new starting lineup. Anyway, so even though I just started the CM last night, I'm currently already in March of my first season, as I hurried up a bit and simulated a lot of matches, since our current team is filled up with so many players I don't like or don't think should be at Barca anymore. So I already went through my first winter transfer window, and I obviously got rid of Raki and Vidal right away, I sold Rakitic to ManU, and Vidal back to Bayern, and bought Fabian, actually swapped him for Roberto, who I kinda wanted to get rid of too, as I didn't see a place for him in midfield, and even though it was harder to sell him than Rakitic and Vidal, since he's La Masia, let's be honest, he's bang average and is now also slowly approaching the wrong side of his 20s, so it's not like he'll improve much for the rest of his career anyway, while I didn't want to keep playing him on RB, even though he would've been fine staying as a backup, but I wanted to give Wague/Emerson that spot eventually. And I also want more for Roberto, cause I feel like he's pretty much got as high as he possibly can, and he won't ever be good enough to be more than a back up player for Barca, while I actually see him fit in Napoli quite well (and I don't see him fitting in a lot of clubs, cause he's obviously only ever played for Barca, but I can kinda see him in Napoli, especially if there's no Fabian there anymore, so Roberto can slot into their starting lineup right away, cause Roberto, even though he won't ever be good enough to start for Barca, he can certainly be a starter for a club like Napoli, so I did this for him as much as I did for myself. And I felt like swapping one Spaniard for another was a sensible and somewhat realistic move as well.
But because I love Havertz as well, and since he's 3 years younger than Fabian, but with an even higher ceiling, I decided I can afford to wait a few years on him, as I feel like he could do with a couple more years at Bayer in real life too. So for now, I decided to bring in Fabian first for some stability and to add some more new age younger Spanish flair, which I feel like is missing at Barca, since Xavi and Iniesta left, and the only Spanish players still getting regular minutes are our seniors, while we lack some young Spanish blood. And I feel like at soon to be 24 years old Fabian is ready to make the step up to Barca right away and slot in the starting lineup alongside Arthur and FDJ right away as I plan on (and feel like) it's time to move Busquets to a back up role.
So my starting midfield (and what I wouldn't mind it all in real life too) for the next few years is gonna be Arthur-De Jong-Fabian, with La Masia gems Puig and Alena with a veteran Busquets behind them to form a pure La Masia midfield (ahh, what can I say, I'm a sucker for nostalgia!) and to play them in lesser matches. And as long as Busquets is still decent, I'll enjoy using this back up midfield. But truth be told, Alena doesn't excite me that much, and I do think that just like Roberto, he won't ever develop into more than a back up player for Barca, so as I like not only Fabian, but Havertz a lot too, I do actually plan on buying Havertz in a few years too, and move on Alena like I did Roberto. I'll probably do that once Busquets retires (in 2-3 years?), to say goodbye to the pure La Masia midfield for good. And with Havertz having a higher ceiling than Fabian, and with him probably developing into a better player than Fabian by that time, I plan on slotting Havertz into a Arthur-De Jong-Havertz starting midfield right away, and moving Fabian into the 4th midfielder role, so he'll still be getting more than enough minutes from the bench, instead of either one of the starting three, and from starting lesser matches alongside Puig anyway. The only question I'm not entirely sure on yet, is who to buy for that back up CDM/CM role to form a midfield with Puig and Fabian, once Busquets is gone. Right now I'm most excited by Arsenal's Torreira, I really like his style and I think Arsenal are using him all wrong, as he's a brilliant defensive midfielder, so I think he might be it.
So my dream Barca midfield in like 5 years is gonna be:
Arthur-FDJ-Havertz as my main guys
and
Puig-Torreira-Fabian as back ups.
With Fabian being the first option from the bench, thus getting the 4th midfielder playing minutes, then Puig getting the 5th, while Torreira purely being there to play when De Jong is not available, or maybe playing CM sometimes, as FDJ is gonna be my main midfielder, playing the most minutes, with him being the only one (with using Puig, Arthur, Fabian and Havertz only as CMs) able to play either CDM (his main position) or CM (when Torreira and De Jong both play). I feel like that's a really well rounded and quality midfield, and I'd love for us to actually have something similar in like 5 years in real life too. Although in real life I hope to see the maybe some new home grown talents, especially the likes of Moriba maybe make it to the first team too.