If you can afford it, buy both. Everything ryaldin and grungi said is true. I'll add that in Icewind Dale, you create your entire party and go dungeon trolling. Very few quests to keep track of, so Icewind Dale is good for getting home from work, and shutting your brain off for a few hours.
The Tales of the Sword Coast expansion adds some fun gaming areas, but is not necessary to the story line. I have enjoyed both games.