Do you use it for expanding your steam library storage or are you also booting Windows? Do you feel 256 is enough and just want more?
I haven't tried Windows yet (but probably will this weekend since they just added fTPM support). I've also been worried about the audio issues, as there is currently no audio driver that I'm aware of (although bluetooth headphones work I'm told). It's all felt very "beta" on the Windows side so far, and I've actually really come to love how easy the SteamOS interface is.
My go-to rule of thumb is to account for Windows taking about 40GB of space these days. The current system I'm on now, the Windows directory is 26GB. Windows likes to download and cache updates and make them available for rollback - which is cool but takes up a bit of space. There are ways you can clean up and consolidate old ones if needed, but it's a pretty slow process and something you'll need to stay on top of more and more the smaller size SD card you'd have. Also, keep in mind that a 256GB card formats as 238.419GB (due to them marketing 1KB = 1,000 bytes rather than 1KB = 1,024 bytes like it's supposed to be) so it's even less storage than you think.
It's all going to depend on what games you want installed at once. You mentioned Lost Ark and I belive that game is about 60GB by itself. Vermintide 2 says it takes 85GB! With just windows and those two games installed, that should give you about 40GB of additional space to play with. Some applications like Steam itself or a web browser will eat into that too.
Personally, I'd go for the larger card. The Sandisk I linked to above also had the 512GB card on sale for $59.99 if you're in the US, which is close to your $50 price point.