I'd be sorely tempted to get one of those Bear Creek SS barrels but they're out of stock. So the nitrided 18" then and one of those Fail Zero BCA's. You're in for 3 bills and those nitrided BCA's are the cat's pajamas for slickness and ease of cleanup. With a nitrided barrel extension, not much if any break in compared to a phosphated (mil spec) BCA and barrel exrension. Just my opinion. Only detraction is 18" vs 20". I'm no bantam weight, so handling a 20" is not a challenge. The only reason I shoot a 20" is because you can't find off the shelf 450b AR barrels in 22 or 24". I like full size barrels, if you haven't figured that out already.
Hoot