Two very, very different festivals.
Werchter usually has the strongest line up on the circuit and it’s the best ran festival I’ve ever been to. Organisation is immaculate, but the downside is that there is literally very little to do other than the music and the stages can get crowded.
Sziget on the other hand, you’ll maybe see 3 or 4 bands a day, only 2 major stages in terms of bands. But there’s just so much else to do, it’s the closest thing to Glastonbury I’ve been to and in a lot of cases it’s better. There’s a lot of genre specific areas, Coloseum is amazing for dance music, there’s a beach for relaxing. The year we went, we got a little passport as a guide for all of the things to do, not many festivals you can go from Post Malone, to Parcels, to a full circus.
I must admit I’ve never been to either since 2019, but two very different experiences and it totally depends on what you want/prefer.
If you need some interesting stuff bar the music, like circus / yoga / cabaret etc. - you should try Sziget.
If you need strong lineup - it definitely loses to Werchter.