I live outside of Seoul in Wonju,a nice city of about 320,000 people. I've made a lot of Korean friends here and I really enjoy the Korean food. There seems to be plenty of "western" food here for me as well, although I'd like a greater variety of spices. There are many foreigners here, but if you're coming to Korea to be with expats, then you'll probably be disappointed anywhere you live.
More important is to find a good school to work for where you enjoy your job teaching and enjoy the students you teach. I've been really blessed with that as well. I work for a hagwon called International English School - Wonju, South Korea and it's the best. We have no problems with pay, we have the National Health Insurance and pension, the tax rate is under 2%, good housing, great students.
www.internationalenglishschool.org/
www.internationalenglishschool.info/
If you only want to be around expats and other foreigners,then maybe you should focus only on Seoul and the other giant cities. For me, these places are just too big, crowded and dirty. Life is much cheaper, you can save more, and the people are friendlier in the smaller cities. There is almost nothing in Seoul I need that I can't find here or have sent in one day free. I can walk, occasionally take a cheap, quick taxi ride and get anywhere. I've got lots of Korean friends. I can get out for skiing and go to the beach easier than I could from Seoul or one of the other megacities. And on those rare occasions when there is something only in Seoul that I have to go there for, I can go by bus or train in 90 minutes. Many things in Seoul are closer timewise from here than for people living on the other side Seoul.
In the end, you have to decide how you want to live. Each person is different.