For urban combat, you would want the CQB barrel. Generally, shorter weapons are better in urban combat (easier to move in tight places and accuracy isn't a huge requirement as ranges are very close).
If you were looking for a general survival barrel with mixed urban and country with possible long range engagements, I would have to go with the standard length barrel as accuracy is needed by mobility is also needed. The long barrel would not be good in an urban environment and the short barrel would not be that great for longer range engagements (particularly greater that 3-400 yards).
I would think the standard would be the way to go. At 16" it's the minimum length you can have while maintaining a legal civilian status (without paying the requisite taxes and doing the paperwork for a short barrel). It's also long enough to provide decent muzzle velocity at the longer ranges.
Since decent normal humans are stuck with 16", that's probably the biggest consideration. Another might be any new ammunition that is coming down the pipeline. Maybe something that is optimized to shorter barrels.