it depends on your intention.
If your intention is to sightsee, then Busan is recommended. If it's shopping, then i recommend Seoul.
Places to visit in Busan.
1. Taejungdae
2. Gamcheon Cultural Viillage
3. Jagalchi Fish Market
4. Yongdusan
5. Busan tower
6. Kkangtong market
7. Orkyudo
8. Beomeosa
9. Seomyeon
10. Haeundae, Kwanganli
11. Dongbaeksoem
12. Haedong Yonggungsa
13. Hwangsan
There's more if you want!
If you need some planning, i will gladly help too!
Do add BIFF square which is a 5 mins walk away from Jalgachi market. Its like a mini bugis street. You can even walk to Busan Tower from there ( i walked from Stanford Inn Busan to Busan Tower and then back to BIFF square for dinner. There is a long stretch of shops that sell food, clothes, cosmetics, etc.