There are a lot of great 18'' SPR barrels out there. I like the 1/8 chromelined with rifle length gas system myself, but some are fond of the mid length gas.
I have a CMMG medium contour chrome lined tube that is sub MOA (Daniel Defense free float tube)
I also have a Wilson stainless medium contour that is WAAA-AAAYYY less than sub MOA also in a Daniel Defense free float rail
I have a Sabre Defense 18'' mid length upper with a Troy Ind. Rail that is coming in the mail it is a 1/7 will let you know how it does.
I have played with the heavy's and the lights and have seen no evidence to support the heay is better than med con.
PK firearms is the best place for SPR type uppers that I have found
www.pkfirearms.com
If you need further assistance let me know.