They're both the same thing, only difference is the new graphics, soundtrack and steam workshop, so It's going to depend mainly on whether you want to play with ASCII or not. Keep in mind that v50 (current version) is lacking some community made utilities and no custom tilesets were ported over as of yet, so if you want to use them you'd have to play v47 (pre-steam version) but you'd be missing out on the new UI and controls which are both a massive improvement. Honestly, the paid version would be easier to learn just because you wouldn't need to decipher the ASCII, but if that's not a problem go with the free version first.

Steam version

Go goblin mode on mods, and apps, and all that crap if you like steam version

I make way fewer dumb mistakes in the steam version. Plus the tutorial is a huge plus for a brand new player

I didn't play much myself, but I believe one thing can be said: if you don't like keyboard-heavy controls, go for the steam version

It has a more modern UI and graphics, even though you could get some of the features (or all of them), you'd need to spend some time setting up thr mods