The difference between a 16" and an 18" is two inches.
Having chronographed 16" and 18" side by side with the same ammo, I have seen nowhere near 100 fps difference. In fact, with 77s it was around 25 fps. If you think that matters, you're in the wrong game.
With 55 gr ammo, I saw about 60 fps difference, which again, is unimportant.
The best argument for an 18" barrel is that you can use a rifle-length gas system and soften the whole system up a bit.
Most 16" guns are carbine gas, but if you're willing, you can get a midlength and it will be a more forgiving system.
Wylde vs standard 5.56 NATO chamber varies depending on who made the reamer.
As far as which to go with, it is really unimportant but the better rifles will be 5.56 NATO chambers. The accuracy will depend on the barrel and luck of the draw.
Most my rifles are SBRs, but I do have some 16" guns and some 20" guns. I skip 18" altogether anymore. I wore my last 18" out probably ten years ago.