First time building AR platform, need advice

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Forged aluminum receivers are stronger due to the fact that they are forged, but the mil-spec dimensions are within a range of tolerance that sometimes ends up with loosely fitted parts. One would need to sit and try many different uppers to find one that is tight to the lower, a matter of trial and error.

Billet is not as strong as forged, but manufacturers can build them with tighter tolerances resulting in a precision fit of upper to lower.

If I was building a competition long range AR, I would choose a matched billet set.
For general use, I would choose forged, due mainly to lower cost.

And , like my DCM rifle, forged lowers and uppers can be tightened up just fine using epoxy bedding and shims, making them as "precise" as billet.
In your case, since you have a billet lower, I would get a billet upper.
There is no guarantee that they will mate up perfectly, but you have better odds of a tight fit, and might be able to send your lower to them and have them find a good match.

FWIW: A tight fit between your upper and lower won't actually affect your mechanical accuracy at all. It's been tested plenty in the past, even by the US AMU and Armalite. That being said--- Some folks seem to derive a psychological advantage by tightly fitting their upper and lower together. It can be achieved with things as cheap as simply placing a foam earplug under your rear lug. Not saying you can't bed or fit your upper, but it isn't really necessary either.