To add my two cents, I'd go for the Atlantic anytime, because that's where the submarine war really got interesting. In fact, the Atlantic WAS the submarine war. Or, to put it the other way around, the only war that mattered in the Atlantic was the submarine campaign. Everything there was developed with the Uboat war in mind - tactics, countermeasures, new weapons and devices... The destiny of a country - the UK - was at stake here and the fight was really fierce, ruthless. That's the reason for the higher interest for the ATO, I believe.
In the Pacific, surface task forces were the name of the game, with aircraft carriers as superstars. Of course, submarines from both sides saw their share of operations, crews fought and died bravely here too, but by no means can we speak about an all-out submarine war here. The very concept of 'fleet boat', i.e. submarines that should have accompanied and protected large surface fleets, proves what were the priorities in the strategic game.
So, in terms of challenge, I guess a submarine game - if done right - would be much more interesting in the ATO, with its wolfpack tactics, BdU coordination and an enemy that gets stronger each year - than in PTO, where US boats hunted alone, against a Japanese navy that put little threat toward the end of the war.
Bottom line, one week from now hopefully we'll know what SH5 will look like, and then maybe all of us will be surprised. The long wait is nearing its end
