I was born in 1970 in the US and to me it's very simple...
War in the Atlantic = U-Boat
War in the Pacific = AC Carrier
It's probably just a natural tendency for a sub sim enthusiast to favor the theater in which the submarine was a more prominent strategic focus.
Another thing... There's not really a US equivalent to the German Type VII.
The US subs were all like the Type IX's. Wasn't really any good short to medium range sub in the US fleet.
There's a post at Uboat.net that I think sums up the differences between the Gato and VIIC very nicely.
http://www.uboat.net/forums/read.php?20,59236,59248
And purely from a gaming perspective...
I think if you like to exploit radar and gadgets, and prefer the stealthy single target ambush at distance. You'd probably prefer the US simulation.
If you're more of a "shoot-from-the-hip" type captain and like to get up close and personal with your targets then the German boats are going to be more fun.
I think it's more fun to play with a Gato in the Pacific than a Type IX in the Atlantic for instance. The real challenge, I think, is getting to the point where you enjoy both sims equally because you've learned to play to the advantages of each.