This is pretty much a physics question, which is fine by me because that's actually something I can actually answer! I will thus leave out the historical details, about the overall effectiveness and popularity among soldiers and whatnot of the helmet, and focus on plain physical properties. The M1 helmet was the US helmet in WWII, and I will compare its ballistic properties to the ACH (Advanced Combat Helmet) and the ECH (Enhanced Combat Helmet), the former of which has replaced the latter.

The M1 had a steel shell that was stamped from a common high-strength steel alloy and was designed to stop a 15 gram .45 bullet at velocities below 240 m/s at a 90 degree angle. ^(Penetration scales with the cosine of the angle of entry, so the helmet was able to withstand higher caliber bullets at higher speeds at different angles) The liner provided no additional ballistic protection. Though this is adequate for stopping a bullet from small pistol and small fragments of shrapnel, it provides little to no protection against heavier pistol cartridges, and zero protection against rifle cartridges, which have far higher speed, mass and penetration. Against a common submachine gun such as the MP40, it would only protect adequately when hit under a significant angle.

The ACH uses para-aramid fibers (better known under their brand names such as Kevlar) which have a better strength-to-weight ratio, and can reliably stop that same 230-grain bullet at velocities up to 400 m/s. That's enough to stop far more powerful handguns at a direct angle, and correspondingly protects against larger and faster pieces of shrapnel. It will still not provide any meaningful protection against rifle fire however. It is a clear improvement however, at a similar weight (M1 weighed ~1.3 kg and ACH 1.2).

The ECH now, uses UHMWPE (a type of polymer that is ridiculously tough) for the outer shell and carbon reinforcement on the inside. I find it hard to get reliable, usable data on the ballistic properties of this helmet, with some sources claiming a 10% increase in ballistic protection, and others claiming it is designed to stop rifle caliber bullets. I find the latter rather hard to believe, but it's safe to assume that it's another major step up in protection compared to the ACH; so full protection against bigger handguns, and more partial protection against rifle fire (at an angle).

So, to conclude with your question:

Which offers more protection from bullets, etc, Murican helmets from WWII or Murican helmets now?

The modern helmets offer considerably more protection, for less weight.