While they all pretty much functionally the same, there are some differences that may or may not be worth the additional money depending on what you feel is important to you.

There are tons of debate between billet and forged so I won’t go into that. However, I find billet receivers to be much better finished. I don’t think that has much to do with the fact that they are billet, I think manufacturers just put much more care into finishing billet receivers. The billet finishes tend to be much smoother, somewhat shinier, and just looks better aesthetically. Even among billet receivers, there are differences. CMT Tac has the best finish, even better than POF, which comes in 2nd in my opinion. Seekins Precision billet finish is nicer than their forged counterparts.

Most of the billet lowers come with integral winter trigger guards. I believe the consensus here is that it’s a must have/upgrade. You can add a billet guard to standard lowers for $18 or an MOE polymer enhanced guard for $9. More importantly, some billet lowers come with ambidextrous bolt release. This is what the Magpul BAD lever is designed to do aftermarket. Having a built-in ambi release though is a big plus for some people. Some billet receivers have flared mag wells so that you can change mags easier. Lancer actually goes 1 step further and allows for users to change the mag well to 3 different sizes. CMT and POF have recessed screws instead of roll pin for the bolt release. This is trivial, but it makes it easier to install and prevent damaging your receiver during assembly. This roll pin is the most difficult to install and is the easiest to damage your receiver. It’s trivial though since there are ways to avoid doing that. Most billet receivers are also enforced in various places where the manufacturers consider to be “weak” on standard receivers. Whether or not you buy into that is up to you. I haven’t heard of any report of broken AR15 receivers due to these “weak” points.

The key difference is actually in the fit of these different receivers. They are all “mil-spec”, but man, do they vary. Some are much tighter than others, but you are essentially at the mercy of upper and lower pairing. It usually works best to pair uppers and lowers from the same manufacturer. Some (CMT Tac, Mega) follow the path of Sun Devil and added a plastic tension screw to allows for upper and lower to fit tight in case the upper is too loose. The Accu-wedge solution is a few bucks, but serves the same purpose. I just don’t like having things other than designed parts placed inside my gun. In my experience, CMT Tac and RRA are the tightest lowers. I judge this based on a non-brand upper I have that won’t fit in either of those lowers. RRA is even tighter than CMT Tac since I can force the upper to fit the CMT, but no chance for RRA.

Personally, I like my receivers to have ambi bolt release, integrated enhanced trigger guard, have nice finish, tight fit but not so tight that I can’t swap uppers, and any additional features that don’t interfere with normal operation.