As far as races go though, the most meaningful for world immersion and unique dialogue are:
- DROW - They get the most unique treatment and dialogue out of all other races. On top of that both Lolth-Sworn and Seldarine Drow each have their own unique dialogue choices which are very interesting and opposite of each other.
- GITHYANKI - Second best to experience different treatment and unique dialogue choices, especially when it comes to a certain companion.
All other races apart from these two are pretty much treated the same, as I've also played as a Tiefling and Wood Elf. Naturally they all have unique acknowledgements from NPCs around the world, but Drow and Githyanki are on a completely different level when it comes to world immersion, acknowledgement and unique dialogue choices.
I'd add Deep Gnome to that list - not quite as much good stuff as drow, but still some great unique flavour in places and unlike drow you can't use the Disguise Self spell to mimic the effects. I've not done a Githyanki run yet, so can't compare that.
With respect to classes, I agree it's not possible to compare though I do feel that the newer classes (druid, sorceror, barbarian, bard) perhaps got some extra love when it comes to specific dialogue and hopefully Larian will go back to the original classes and flesh out their options.
Then of course background can add to flavour as well, as you'll get different inspirations/rewards, so there I'd suggest avoiding the backgrounds your companions have to maximise the variety you see, though not at the expense of not creating the character you want.
In general, though, I'd just agree with the advice to go for it and not worry too much. It's only early access so you're not going to be stuck with one character for hundreds of hours and can even try a few to see how that changes the experience. In most cases, there are different ways to get the same info or outcome so you're not missing out on vast swathes of content by picking one character over another, just nuance and flavour. I'd just pick whatever most strikes your fancy.
