You can use either option, Glad2bnuts. A forged or billet receiver will afford the most ruggedness, but they are more difficult to machine and require the use of more advanced tooling (a drill press or router). Forged receivers are actually easier to machine than billet aluminum, though billet receivers have their own benefits (threaded pin holes instead of using roll pins and punches), and the trigger guard's usually integrated into the receiver blank (
example here). A polymer receiver can actually be completed using a Dremel, hand files, and a handheld drill. You can still use a drill press if you want to. The only downside to using a polymer lower is the included jig is usually "eaten up" in the process (it, too, is made from polymer to aid in easy machining). But the whole process is safer and I'd recommend it for first-time builders who want to get their feet wet.
I wrote a guide on finishing polymer receivers, too.