So here's the gist of it:
Pretty much everything that the Enhanced Editions do for Baldur's Gate there are mods that also do. Higher resolutions, sharper UI, new companion characters, added dialogue, quests, etc. Thing is that's a lot of modding, moreso than some people want to have to do, so for those that would prefer to just install and play the game the Enhanced Editions are honestly just fine. They were a bit buggy on release and weren't compatible with some of the best mods available, but most of my favourite mods have since been ported over and the devs have done a respectable job of patching the EE up to a higher standard of quality. The
BG1 NPC Project mod for example I consider amazing and has been nicely ported over to even take advantage of some of the new stuff the Enhanced Editions provide.
So if you're feeling up for some tinkering and want a more customized experience modding the original base game is the way to go. If you want a more simple install and play approach then the Enhanced Editions are your best bet.
Either way Baldur's Gate 1 and 2 are amazing games, some of the best rpg ever made in fact, regardless of which version you play.
I have not tried this new expansion yet myself and can't speak for its quality however, I'm talking purely about the original series here.