I played the first one more because it was "novel." Other than that however, I do believe that Borderlands 2 improves upon just about everything in Borderlands 1, and had I played 2 first, it would be my favorite.
Yellow stuff are not hard to obtain in BL1 but weapon scaling/level is just that crap; you managed to get a great lvl 32 weapon from pt1 and wonder why that weapon is still better than those lvl48 that you have just found while you are on pt2.
True, the second has better graphics, music, UI (debateable), character development, etc, but that is expected from a sequel.
The first Borderlands was a hell of a lot more fun. It FELT better to play. There were far fewer sequences of endless hordes of enemies, and more strategy was involved in playing. More fun, better story, etc; I liked the first one better.