I'm not so sure it's possible to "decidedly" distinguish between the two - especially now that we know BG3 is turn based.
I think that people will always compare BG3 with D:OS2. Games are so close in style that the mentioned distinction won't be big enough for BG3 to stand independently. Fans of D:OS2 will be very happy about it, fans of BG series not necessarily.
- D:OS had good co-op options that people really used. BG had co-op of some sort but I don't know if anybody really used it much.
It may be a wonderful experience if the co-op function will be at least as good as the one in the D:OS series.
- BG had way more spells than D:OS.
That's the advantage of D&D. I'm not worried about this aspect of BG3. From what I've seen in the gameplay, Larian wants to implement D&D ruleset quite faithfully. That's why I think the game may be the best implementation of D&D in history of video games.
- BG had Minsc, the best companion NPC ever (IMHO). The rest of the NPCs were pretty average, though.
I think that there are a few more fantastic companions in BG series. My personal best from BG2 apart from Minsc are Viconia and Jan Jansen.
- D:OS has better music.
It's really a matter of taste. Both are great IMHO.
So, if they take their spells from whatever the latest D&D is and channel their silliness into one great NPC, then I think they are set. Being turn based and having better co-op actually matches D&D better. I certainly don't want them to make average music and the ability to pick your companions' classes really is a wonderful thing.
As of know we don't know whether the player will have an option to choose a class for the companions. I still believe that BG3 will be a good game, but the question is whether it'll be a good BG game. For now I see more BG in NWN2 than in BG3. Larian still has a lot of work to do.
P.S. Oh jeez, I forgot a big one. Resting! In BG you go into a dungeon, fight through a few rooms, and take a nap for a few hours right there in the hallways so you could re-memorize your spells, heal up, and so on. I don't care if the solution is the Divinity teleport pyramids, shorter dungeons, or something else - just don't make me endure the silliness of pitching a tent in the middle of hostile territory!
I agree that this was annoying. The system favored save scumming. If the rest was unsuccessful most people automatically loaded the game to try again. Not much logic in the Infinity Engine games rest system.