It's difficult to choose between them, in terms of stating categorically that one is better than the other. Disregarding engine and graphics changes and basing my decision purely on the story, in-game events and player options, i.e. choices of any kind, from which NPCs to take along or how to solve X problem, I *still* can't make an easy decision between the two. >laffs<
The stories in the series is a good one, compelling and enjoyable. The two games introduce, carry on and ultimately conclude the story in a very satisfying way, IMO. The two games *are* different, if only with regards to party level and the effects of that.
BG1 is more of a "growth" game - you start off very weak and underpowered and probably not a little confused about just what it is you are meant to do, with regards to the Big Picture. However the game is scaled to your level and this helps a lot - there are few areas that you can access at low level that are suicidal [and if you do get somewhere insanely deadly, re-load and don't go there yet!] Even low level monsters are challenging and it takes awhile before you can become more confident in your abilities and survival. I *love* this aspect of the game!
The fact that your weaker characters *are* weak is a great game play reality. You have to choose your battles and tactics wisely, you have to learn how to use your skills and spells effectively and you have to accept that simply running away is not only a viable option, but sometimes the *best* option.
BG1 is even less linear than SoA, which is itself pretty non-linear in that you always have side quests, party NPC quests and random events giving you a lot [sometimes it feels like an overwhelming amount] of options. Each game in the series gets more linear as you go along - BG1/TotSC least linear, SoA a bit more linear and ToB really very linear.
For me personally, I like mid-range level adventures best - so late BG1/TotSC and much of SoA. By the end of SoA and all through ToB my interest level flags - the battles are routine, the tactics are 95% set in stone and the enemies *all* have to cheat to remain the least bit threatening. My attitude here is shown by the fact that I have completed BG1 and BG2 multiple times each with a great range of player characters - I have finished ToB just twice, and mostly just so I could say I had done so - I have very little interest in finishing the entire series again, although I'm very much enjoying my two concurrent games of SoA with different PCs.
Even though they share much the same engine, one single main story-line and pretty much the same in-game rules [ADD 2nd Ed - vastly superior to ADD 3d+], the games are noticeably different in content and play - not significantly different, but noticeably different. I would give the edge, slightly, to SoA/ToB, but not by much. If I *had* to choose having just one, it would be SoA/ToB over BG/TotSC, by the slimmest of margins.
However, it is a very small margin! :) I love BG1/TotSC for its own components and having to think about how you are going to run a battle against Hobgoblins, while actually feeling fearful for your party's survival, is a blast! Facing the chance of having your entire party eaten by a single Black Bear will create a battle which is, for me, massively more enjoyable than wizzing through multiple Fire Giants in ToB or hordes of Trolls in SoA. The sense of possible doom in BG1/TotSC is, for me, much higher [and thus more exciting and satisfying] than anything in SoA/ToB, apart from the big "major boss" ending battles therein.
However, as it stands, I have access to all of the series and even though I've completed all the games [and we are talking 10 or 12 years ago] I'm happily playing them again! Since we don't have to choose, I won't in actuality, but for the purposes of the question, like I said, I'll rate SoA/ToB slightly, say 2% - 4% points better, than BG1/TotSC.