Now...you're on a BG2 forum...so, of course you should expect to have everyone tell you that it's the best game that was ever created.
With that said...here's my take...
If you want all around good RPG gaming, BG2 is the way to go. Personally I got bored with Diablo because it was just always more of the same. Find monster, kill monster, take loot, find more monsters...etc. BG2 requires you to think and solve lots of odd quests and puzzles. It is very "replayable" and is a little different every time you play it based upon your character's chosen field of study (Fighter, Mage, Cleric, etc.) and who you choose to include in your party.
Overall, money well spent...but you may become a bit addicted and forget to eat/sleep/work, etc.
However, if you are simply looking for a hack 'n slash dungeon crawl...go with Diablo or Icewind Dale which is also a good find monster, kill monster, take treasure game...so I am told.