Most brands are the same when it comes to quality and fit.  Very few of the AR makers actually machine their own lowers and uppers.

Billet looks cooler to me, but if you speak to a metalurgist it's probably a little weaker.  But not to a point that you'll ever notice without expensive equipment to measure it.
Forged starts out as a block of aluminum that is pressed into it's basic shape then the machine shop finishes removing all the metal that doesn't look like an AR.
Billet starts out as a block of aluminum then a machine shop removes all the metal that doesn't look like an AR.
Billet is more expensive (unless you are just running a few pieces) and has no actual advantages other than looks IMO.
Forged is cheaper, but has no other actual, practical advantages IMO.
I go forged just for the price.  If it were as cheap I'd go billet since I tend to like the more angular look of most billet.

And like I said, most every lower is of similar quality.  I won't pay $130 for a stripped lower when there are cheaper, equal quality lowers available.  I won't go cast, and there are a couple companies I'd stay far away from though.

Short version:

Think of it this way:

All metals have a grain structure to them...vaguely similar to wood.

With a billet lower, in the process of machining a lower receiver, there are many opportunities to "cut across the grain" as it were.
When that happens, there are inherent weak-points where the possibility of a stress riser can occur.

In an attempt to counteract such problems, they thicken the lower up where they can in order to increase the modulus of elasticity (essentially stiffness).

In a forged lower, the billet block is heated, and then further forged in several steps of forging and finishing dies to achieve the final shape, and then through a final heat-treating.
This process essentially takes the grain structure and forces it to conform to the basic surfaces of the lower receiver.

This minimizes the chance of any internal stress risers occurring, and further increases the material density beyond that of the billet's state.
That allows the manufacturer to use less of a (essentially stronger and more durable) material.

Basically less of a denser material, or more of a material that is less dense...six-of-one, half-dozen-of-the-other to me.

The only way to really see a noticeable difference between the two would be in the abuse that a firearm would see in daily combat over extended periods of time.
Likely why the .mil prefers forged uppers & lowers.

Myself, I own both, but prefer forged...though forged is not as pretty as billet.
Just my $.02.