I think either will stand the test of time and both are classic, but in different ways. I made my BB decison when I saw it on the wrist of a business man wearing a crisp white shirt. Incredibly clean and polished look. Not fun - elegant, professional. But if you get too large a size, it can look too fun/trendy, IMO.
If I was in your shoes, since you have a round watch already I would go with the Tank for shape or color variety. Both beautiful watches, so you can't go wrong. Pick the one you like the most so you have no regrets. You posted two pics of the BB, so maybe that one is your fav since you can't stop looking at it and you're just considering the Tank because it seems nice since so many here have it, but it seems safe. Buy what you want to wear today.