The main advantage of building your own is that there is (in most states at least) no official record you have it. Anyone interested can track your credit card history to find associated online orders, but there is no ATF Form 4473. Secondary advantages include the possibilities of a better understanding of your firearm or of better quality parts.
Personally, I don’t see a difference between a $1500 kit and the corresponding $1500 complete firearm. As for picking parts one at a time, that strikes me as far more expensive than a comparable kit or comparable firearm. Perhaps someone else has done better, but I haven’t figured out how.
The “lower” is the only part that has to be delivered via an FFL from what I’ve heard. All other parts can be delivered to my front door. If the money is about equal I would buy a complete weapon and wait to learn how to “build” one until I leave this terrible place that is New York State.
That is only true if you complete an 80% lower. A stripped lower must be sold on a 4473 from a federally-licensed dealer.
Unless you buy an 80% lower and finish it yourself, the lower will have a serial number and require a 4473. The feds wont have a record of that specific configuration, but will know you are in possession of a lower receiver.
CC