The thing is, nobody can really tell you where the price is going in 6 months or a year. Both networks are working well, it's not like one is clearly "better" than the other in 2026. Solana does have fewer congestion issues, but that doesn't automatically make it a better investment - technology and utility are one thing, price is something completely different.
What you should really be asking yourself is: are you comfortable seeing your investment cut in half in a few months? Because with crypto that's possible, honestly. People who tell you "it's ultra volatile so be careful" are right on the fundamentals, except it's true for both. ETH has a bigger user base and more apps built on it, so there's an argument for saying it's "less risky" in theory, but crashes hit the whole market the same way in the end.
If you really have to choose and don't want to think about it every day, maybe split your investment and go with both instead of torturing yourself trying to guess which one will perform better. €500 is an amount where you can really afford to diversify a bit. Otherwise, read up a bit more on the concrete developments for each network over the past few months and see which one inspires more confidence in you - often that feeling is the most useful thing.