I'm not sure which to get, which do you think is the best?

Different D&D rule sets are used for each game. I personally prefer the first, although the second is a great game as well.

I have played one over and over - yet to finish two

Both are extremely linear hack 'n slash crpgs.
I think I'd go for the first one, even though it's older it is more fun to play.
Plus i think in the case of the crpgs made by bioware at the time AD&D rules are better than D&D 3rd edition ones.

Potentially irrelevant correct: It was made by Black Isle.

I prefer the first one. 2 is not bad too, but IMO IDW1 has more soul in it. If you can afford it buy the first and when finished the second. But be warned, No. 2 is insanely more difficult compared to the first one and makes the reload button its most memorable feature (again IMO).

Edit: Cocky sidenote. IIRC IWD2 used a strange hybrid of AD&D and D&D3.0 because of engine limitations.

IWD 2 was easier then one for me