Buying one already built can be great if you have a good warranty for it and any issues come up. 308ar can be finicky because everyone builds them different, no set standards unlike ar-15s. but even if you get a finicky one, a little reading or questions on here, can get you up in running in no time.
building one can be good if you don't have the money to throw into a rifle all at once, or if you just want to save a little money by finding really good deals. Plus you can't beat building a rifle that is exactly what you want from the start. and you will learn more about the rifle from building one than you ever will from owning and using one for 10 years.
Either way you chose, good luck with a great platform. You can't go wrong with AERO and many other companies. 🙂