I have a CMMG 18" SPR barrel with the rifle length gas system. It's very accurate and very smooth cycling (very reliable too).
Your choices will be rifle, intermediate (on those who offer it) and midlength. Midlength is likely to be a bit overgassed, with rather strong extraction and ejection, sometimes overly strong. Rifle might be a bit undergassed (mine isn't), but that's easily enough handled. Intermediate might be the best pick, but t'were I to do that, I'd be likely to go with a low profile gas block because handguards will be hard to come by- the low profile block will let you use a rifle length free float handguard, increasing options.