Rifle or mid length gas system on 18" SPR barrel?

I'm also inclined to the rifle length system. Most suggest this just works better because that was the original design. Run a 15" Samson, Midwest, etc., over the block and you protect everything fine.
 
I've used RLGS on all but one of my 18" SPR-type builds.

The MLGS 18" upper is meant for extreme cold weather use, but I see a lot of 18" RLGS SPR's run fine in those conditions as well.