Define what you want the rifle to be used for, and make your decision based on that.
If you're looking to build a Mk12 SPR clone or a target rifle, go with the SPR contour (41 oz) and don't look back.
I once built a "lightweight SPR" around a LaRue PredatAR 18" barrel that weighed about the same as the WOA Mk1.9 contour (around 30 oz). It was a nice starter gun and served me well as I began to shoot more frequently. But as I began to improve and hunt more, it felt like a bit of a compromise. Groups would open up after a few short strings of fire at the range due to the light profile, and the 18" barrel was less than ideal when trudging it through brush on long hunts (I'm rather short). I eventually learned that rather than 1 compromised rifle, I wanted 2 focused rifles that were built to suit more specific roles . So I sold the LaRue and built an 18" Mk12 SPR and a 14.7" Recce that do a better job at their intended roles.
I must also admit that while the 18" LaRue's midlength gas system didn't bother me at the time, I was really impressed by the soft recoil impulse afforded by the 18" Mk12's rifle-length gas system. I definitely prefer using the longest gas system regularly available on just about every barrel I build around now.