The M&P Sport II vs Ruger 556 is a fair comparison. It's the lowest offering of both lines by quality manufacturers. They are basically the same rifle: 16", carbine gas, entry level. If you're going point for point, make sure you compare the Ruger #8500 to the S&W #10202 (or just make sure you are comparing equivalent models).
Both offer upgraded furniture under different model numbers.
Ruger has a few basic addons, like the grip, QD socket on the FSB, muzzle device is their own - not exactly an A2 birdcage, but not different enough to matter.
M&P SII has plain jane furniture.
1:8 vs 1:9. This doesn't matter enough to tip a decision point. As much as people like to advise bullet weights for certain twists, the individual barrel will have preferences. I have seen 1:7's not shoot 68-75gr well and actually prefer 55gr. Go figure. One of the first uppers I ever had was a 1:9 Rock river HBar
Nitride vs CHF: I noticed that the Ruger is advertised as CHF, but not chrome lined. Sport 2 is nitrided...... oh, I'm sorry. It's armornite'd - hahaha. This decision point is a wash also. If you ever intended on shooting this upper full auto, like 0.1% of America, you wouldn't be looking at these two rifles. If you ever actually get the round count on either that is high enough to break a bolt, either would just go back to the manufacturer for service.
Semi auto BCG: I would have zero concerns over this. Manufacturers have been producing them for years. There are no epidemic problems or recalls. Those who want to pound their chest the hardest that they have a FA carrier typically aspire to an operator lifestyle that will never will never be attained. They wouldn't be looking at these rifles either.
My reference point is the Colt 6920. How do they compare to the Colt?
The 6920 is near the bottom of Colt's line, but is the better rifle. The Colt upper will run full auto. It's chrome lined, but not CHF. It's 1:7 twisted. Plain jane furniture. Not trying to be a jerk, but the Ruger or S&W are more likely to have better accuracy than the 6920 with typical 55gr ammo.
Final thoughts: Just being honest, I dislike the 16" M4gery - haha. How long has it been since you've heard that term! The evolution of the AR makes all three of these rifles look old. I personally would spend more on something that has better features. Like a free float rail instead of staring at a FSB when you put an optic on it. Like 16" midlength gas or 18" rifle gas for smoother operation. Like something that may offer a discounted upgraded trigger. Like something that offers or already has upgraded furniture.
Of these two, the answer to the how do they compare question is that they are basically very similar.
