IMO Seomyeon is the best area for Busan. That's because it's right in the middle of everything. Busan is very spread out. So Seomyeon to Haeundae is 45 mins-one hour one way by subway or so. Seomyeon to Nampo-dong is like 15 minutes and easy.
But Haeundae is VERY far east, so you'd be traveling a lot back and forth if you go many places in Busan. So Seomyeon IMO makes sense as it's central to everywhere and balances the travel times overall.