The Sport II is an entry level rifle, the barrel isn't chrome lined the handguards don't have heatshields, and in general it's a bare-minimum rifle. The colt is ~another $200 and for that you get basically an overall higher quality gun that will run harder and longer. Now, if you're not the kind of person that worries about trading bullets with bad people, or at least values that ability in a gun, then you won't notice the difference and don't need to spend the extra money.
What parts do you want that generally don't come on a factory gun? What do you want to use the gun for? I suppose those are going to be factors in buying vs building.
Just about anyone can put a gun together that works at the range. I've done it with less the the recommended minimum tools. I don't think I really want to trust that particular gun for anything important and I know I didn't get a better gun than I could have bought for the same money at the time, maybe as good. It was a good learning experience so in that regard, go for it. But with AR's where they're at, I would say most folks are better off just buying a gun with a warranty. A S&W Sport II is ~$650 and MAYBE if you spend a month or two doing the research and carefully selecting parts you can build roughly the same quality rifle for roughly the same money, and that's just the cost of parts and tools. Doesn't factor in the time involved. Time you could spend enjoying your new gun or with your family or anything more fun than in front of a computer screen figuring out what parts to buy and how they go together.
