My impression so far:
While the USN have better 'on paper' AA stats, I found I was killing similar amounts of aircraft on either. AA is kind of hard to make effective; good CV players usually don't have their planes fly directly over cruisers. The only time I seem to shoot down a decent amount of planes is either the planes are bombing me, in which case, they usually die after they've already dropped their load, or if they're retreating after already bombing our BBs. So in reality the AA seems pretty much comparable - not really game changing. Torpedoes are so overpowered, that these are actually game changers. When you manage to fire a broadside (Senjo has 15x 610mm long lance broadside - each boasting about 20k damage) and hit a BB and sink it, their team suddenly has 1 less BB and the tide of battle turns in your team's favour. Torps can also be used to force ships away from an area, and make the player so distracted trying to dodge them, they are suddenly not firing or are less accurate.
No game was won by shooting down 1 or 2 extra planes due to your elite AA. The Des Moines, though, has 12x2 76.2mm/50s which have a range of 5.7km (base, without skills or mods), which would probably be able to extend the AA pretty much over half your team - this ship would be the ultimate in AA. Bear in mind, these are Tier X, so take a while to unlock as well as you're up against equally skilled Tier Xs.
To sum (just remember, there are plenty of exceptions to these)
USN mid tiers have excellent AA and guns
I find are more bulky and slower, with slower turning
Slightly more hitpoints
Armor seems the same as IJN, with some exceptions
Aesthetically appear more unique between each ship - the New Orleans looks smexy. Baltimore and Des Moines look the same, and are square/boring to look at (IMO). I do like the blue paint theme though.
IJN seem to handle better and are somewhat faster,
Their guns are slightly less powerful, however, generally have more
Torps are overpowered, with a low skill level to use.
The AA seems to, in reality, shoot down similar amounts of planes.
Aesthetically similar, longer, smoother curves. Senjo looks sexy. Most of the others are kinda ugly
I always hate playing the same class or tank/boat as everyone, so usually opt for the underdogs. Some battles there's 80% USN cruisers, others there's 80% IJN cruisers - can't tell which nation is played more at this stage. I've seen no Ibuki, but I've seen heaps of Baltimores.