I don't think you will find an 18.5" barrel factory threaded for choke tubes. There are a few 20" rifle sighted barrels that are threaded, my brother owns one. I have a 21" turkey barrel so threaded and a 20" rifle sighted barrel with fixed IC choke.
As an all around survival shotgun any of these would be my personal choice. A 20-21" barrel is short enough for SD. Having interchangable chokes makes the gun more versatile. A Skeet or IC tube is about perfect for SD purposes and other tubes can be used for hunting. Actually the fixed IC barrel shoots birdshot into tight enough patterns for most hunting.
The 18.5" cylinder choked barrels are fine for close range SD, but lack any versatility, and usually shoot buckshot into wider patterns than I'd prefer. But if close range SD is all you intend to use it for then that barrel length would be a good choice.
A longer barrel is clearly a better choice for hunting, but I'm talking about a survival gun. I'm not interested in giving game a sporting chance and would only be taking easy, close range shots at animals setting on the ground or water. In a survival situation I wouldn't be wasting ammo on low percentage long range shots at flying birds.