I assume you mean Seomyeon?
Busan has numerous areas where things are "happening" as far as foreigners are concerned. Getting around is easy enough, public transportation is cheap and efficient, and though taxi prices just went up 20% they are still rather cheap (6,000won ride from Seomyeon to Haeundae for example).
The highest concentration of foreigners is in Jangsan, in Haeundae district at the end of subway line 2. But people live all over the city as well.
For going out:
Kyungsung Uni, PNU (Pusan National University), Seomyeon, Gwangalli & Haeundae all are popular. Lots of new bars are opening up to cater to foreigners, so things are shifting somewhat, since PNU used to be the top spot in the past (well before I got here), but Kyungsung is pretty happening as are the beaches (especially in the summer).
Kyungsung is on line 2, has the city's only techno club (Vinyl Underground on Saturdays) and has a bar-filled area of a few city blocks. It's popular with Koreans and foreigners, and has several well-known bars including Thursday Party and Ol' 55.
PNU is on line 1, on the north end of the city, and has 6 foreigner bars in one area, as well as tons of bars and restaurants all around. The Basement and Soul Trane are probably the most well-known. Lots of bands play on Saturdays...
Seomyeon is a major shopping area. It has a vast underground market, plus tons of Western restaurants and shopping above ground. It has a few bars, like Fuzzy Navel and Foxy's, although I actually haven't been to either of them :-).
Gwangalli is a beach, accessible from Geumnyeonsan on subway line 2. It (oddly...) has 2 Thursday Party bars right next to each other, on the main beach road.
Haeundae is a bigger beach, crazy busy during the summer. Not so many bars in the foreigner scene, though yet another Thursday Party just opened a little while ago. Went to a new 14th floor bar this weekend, called Rock N Roll House that was pretty decent. U2 is a well-established bar a bit off the beach...
So, back to your original question of which is better, kinda all the same... Seomyeon you'd be better located, more centrally, so going out to the different areas would be easier and cheaper, and being on a subway transfer is quite nice. I live in Yeonsan, another transfer, and its definitely nice for getting around. Most people here don't go out to the same area twice in a weekend due to lots of barhopping, so its common to do PNU on Friday and Kyungsung on Saturday, or whatnot.
Haeundae is nice because it has lots of foreigners (if you want that, I like living in a Korean neighborhood, after all I'm in Korea....) and foreign restaurants and such, but you would get that in Seomyeon as well. The beach is nice in the summer, but summer here is pretty literally 2 months (July/August) and Haeundae is packed the whole time.
Hope this helps, if you more questions just ask.
Check out pusanweb.com / koreabridge.com if you haven't already found them - they have lots of information and resources about Busan.
Peace
Anderson
