Is there any real difference in 223 wylde 16" and 18" barrels?

Less difference than going from 14.5 to 16. If you aren't going to be doing a whole lot of "carbine" type stuff, I'd say go with the 18 for a small but measurable gain in velocity and less muzzleblast. I guess that's mainly how people are differentiating a Recce from an SPR. Of course, barrel quality and twist makes bigger difference than 2 inch of length. I went with a 16" stainless Raineer match 1:8, and love am loving it. I'd probably jump to a 20 or 22 for a longer range upper.