Mainly it comes down to whether or not you want to use the pom poms, if you do, you need an AA gunner, if not, reload gunner is fine.

I classed my UK AA gunners at level 90, this gives them very good reload stats and gives you the bbenifit of being able to use the A guns and pom poms over Reload gunners.

Only class to 2nd AA gunner as well, as the final classing isnt worth the loss in reload.

Also, the UK AA gunners can now use HH.