Start with 64 bit.

Note that Steam is not entirely stupid. If you later create a 32 bit profile, you can just symlink the "common" directory (or specific game directories). Steam won't immediately see those, but when you press the install button it will recognise something is already there and verify first. This won't use much data.

I suggest you to create both prefixes and install two Steam clients: the vast majority of my games are 32 bit, but some are 64 bit only. i.e. for years I believed the game "Betrayer" was broken, then I discovered that it is a 64 bit only game ;)

I am not sure about Steam, bit there are multiple cases about software having problems with Wine when using a 64 bit wineprefix, which problems are not present with the 32 bit wineprefix.
As far as I know, the Steam client itself is still a 32 bit software.

Thanks everyone. I went with having both, although I'm installing into the 64 bit drive. Fortunately, I don't have many Windows only games. Most were part of bundles, or some platformers I bought for my wife.

Yes, it looks like Steam is 32 bit, although I imagine it shouldn't matter too much as it's basically a downloader and launcher.

I also did a bit more delving around. It looks like my ISP runs a CDN that caches Steam content, so if the games I install are cached there they aren't counted against my quota. Happy days. I'll finally get around to installing some of the larger native supported games I have in my backlog, like Alien Isolation (~30Gb) and Mad Max (~34Gb). :)

Quoting: EhvisStart with 64 bit.

Note that Steam is not entirely stupid. If you later create a 32 bit profile, you can just symlink the "common" directory (or specific game directories). Steam won't immediately see those, but when you press the install button it will recognise something is already there and verify first. This won't use much data.

No need for external symlinking. In steam under settings, downloads add another steam directory that points to the 32bit steamapps folder. This way Steam finds the games directly and is not confused about validation etc.

Regarding 32 or 64bit: so far I have only found the new Doom 2016 that requires 64bit. So its probably better to stick to 32 bit for most games.