For an AR build, 556 223
This ^^^^^My usual pitch on them, having minor experience with both, is that forged is stronger by volume due to the forging process and is cheaper due to the reduced machine time needed to finish them. Billet is more interesting to look at, and is often built thicker in known weak spots in the AR design to resist or at least reduce flexing but is more expensive since there is a increased machine time production component of the pricing. Billet is usually heavier than forged on a per-piece basis.
The strength argument is usually ignored by pointing out that lowers are often found in plastic, with the old Cav Arms system being the strongest (and lightest).
Like many things in life, and especially in the gun world, the difference boils down to personal preference
Alex
Billet Lowers and uppers are coming down in price. I picked up a billet lower from a start up company for $125. I spoke to the owner and they use all the same processes as say Wilson Combat or any of the big names, like most things it's the name you pay for.
I do actually have some background in machining and manufacturing, and that makes me doubt their claims. I'd like to know if they're using billets with the same qualities of the big names. One example: Are they taking a cast block and machining it? 'Cause that'd be very different from what Wilson et al do, but give results that the layman would never be able to tell apart.