what gun are we talking about?
do you already own the gun or are you looking into buying an 18'' gun for turkey hunting?

i have an 835 w/ a 20'' barrel. it seems hard to find an 18'' barrel with interchangable chokes.
A good friend won an 870 home defense gun w/ and 18'' barrel in a raffle, then later on whenhe decided to get into turkey hunting and sent his barrel to briley and had them thread the barrel and used their full choke and it shot great. he still hunts with it. thats the only experience I've had with an 18'' turkey gun.
Basically any 18'' barrel with a good choke is going to get the job done, the trick is finding the right shot shell for it.