I'm a yank (damn proud too
) and SHIII helped me appreciate U boats even more than I already did. I love them, I love their stories, and I still love SHIII. That said, SHIV has helped me to appreciate our good ol sub boys as well. Both sides is VERY interesting if you only read into it a little.
Either way I will keep playing both. The gameplay of SHIII is just awesome, but there are some gameplay elements of SHIV that is great as well. Once the modders fix the other problems, I may start to have a hard time coming back... Who knows.
I do think SHIII's days will be numbered when, or if GWX moves on. I know GWX is what has kept SHIII alive for me at least.