Take the IX, if it's only possible. It's an award to get such a boat as one of the first commanders.
I say "take it" not because it's better than VII - I just think you should have a try so that you learn which one you like more. It's a matter of taste in my opinion, and maybe of style of play. There are a few around who love type II boats and use them even late in the war, just because they're their favourite.
I remember an EU slogan: "Celebrating diversity". :rotfl: