I would absolutely love to go to all the Nordic and Baltic countries someday! I really want to explore more of France, Germany and Belgium too (I've been before) and visit the Netherlands, Switzerland, Austria and the Czech Republic, though I'd class all those as Central rather than Northern.
Despite the above, though, I would still pick the Southern European countries if I had to. As standreams said above, although for example the Nordic countries obviously have differences, they also still have more similarities than countries like Spain, Italy, Greece etc. do between each other, and indeed countries like Spain and Italy differ widely within themselves too. Being from the UK, we have more similarities with other Germanic countries, so Southern countries seem more exotic. I love dry heat, too, I find the history, art and architecture more interesting, and I love the food more (though I can't say I've really tried any Nordic cuisine, and I'd love to give Czech, Hungarian and Polish cuisines a go - I think I'd really like them! Really wouldn't class them as 'Northern' though...)