Between the two version it asks you to pick from at the beginning of the game. Also, what's the differences that Difficulty levels cause? Just more HP for enemies, or does it affect more?

The difference between Complete and Chronicles is just the artwork (as in the dialogue sprites, store screens etc.). Complete's look should be familiar if you watched anime around the turn of the century and Chronicles's is more modern. IIRC there are also some differences with the cutscenes. Fortunately you can switch between the two modes at any time in the PC version.

Note that before you even start the actual game, you have a similar choice in the config tool. This one affects the aspect ratio. Complete is 4:3 and with a frame around the play area, Chronicles is widescreen without the frame and zoomed in, so the visible area is smaller.

Higher difficulties affect enemy HP, damage Adol takes, damage Adol deals, experience/gold gain and probably something I've forgotten.

Thanks, mind I have no beaten both of the first two Ys. Not bad, and I actually kind of dig having the Touch of Death ability. Quite interesting. I am aware that later games are more traditional in that regard, however, sigh.

You may have to swing your sword, but they are still really good and feel a lot more like the "good old action RPGs" than anything else.

I do enjoy a good Action RPG. Probably gonna do Origins next, then Oath, then Ark. That seems to be the agreed upon best order.

Well yeah, because the story order is:
Origins, Ys I, Ys II, Oath, Ark

I strongly disagree; when a book, movie or video game is released as a prequel, it doesn't mean it should be read, watched or played in chronological order. In fact, prequels often make references to the media that was previously released and readers/viewers/players miss those if they decide to read/watch/play a prequel first.

My suggested order to play the Ys titles available on GOG is as follows: Ys I, Ys II, Ys III (Oath in Felghana), Ys VI (Ark of Napishtim) and only then Origins, the *prequel*.

From what I've been told, Origins is mostly referencing the events of the first two games, as it's about the land of Ys itself, so those are really the two that get the lion's share of the plot. Does it reference stuff is Oath and Ark that much?

I personally prefer to play in story order (also hate it when movies show the ending first and then jump back to the past).

And I don't know anything in Oath and Ark that Origins refers to... they are mostly standalone stories and the only premise you have is "You are this boy named Adol that really REALLY likes adventure."