GTA 5 Steam vs. Rockstar normal key?

If rockstar doesn't want the game to be modded it's safe to assume there won't be a workshop for it on steam either, the only real benefit is steam achievements since you have to add friends in Rockstar social to be able to play together as far as I've seen.
Also, the retail (boxed) copy is a rockstar social key, not a steam key.

the only difference is that one shows on your steam account and the other one doesn't
there's no advantage
it doesn't have support from steam workshop

retail is rockstar

Steam is very useful for keeping the game updated and being a source of the download, should you ever reinstall GTA V. That's pretty much all that you get from Steam version, everything else is presented in retail. But when you buy GTA V on Steam, you also buy the retail key, which you can use without Steam, which doesn't work the other way.