Which edition should I get?

[Funky Kong]

I'm thinking of grabbing this on sale, but I want to know if the edition you purchase actually matters.

[Hopit]

Commander. (cheapest one)

The more expensive ones don't give any gameplay benifits (apart from a few coat of arms, but they're just cosmetic)

[Hawke]

I'd suggest to go with Commander and purchase an upgrade (to Overlord or Archon Edition), if you like the game.
The artbook and the OST are quite good.

[Dorok]

I second, start with base edition, other editions are really for fans only. And as it's a special RPG with an uncommon blueprint, there's a significant chance you don't end be a fan or not that fan even if you enjoyed.

The only problem is you'll have to wait another sale for the upgrades and right now only Steam sell them, so it lower chances of sales.

[D'amarr from Darshiva]

Easy. You choose what you want to get. Here is the comparison table:
http://www.paradoxplaza.com/tyranny/TYTY01GSK-MASTER.html