I am going to apply for a Non-Immigrant B (Business) visa next month. There are two options for me at the moment: I could get it in Korea, in my home country, or in Singapore.

1) Does any one know if there are any problems/difficulties getting this visa in either countries?

Both of the embassies claim that they issue a 90-day single entry visa and the visa could be extended to a 1-year multiple entry visa at the immigration office in Bangkok.

2) Has anyone done this? Is there any problem with this?

I got a 90 day single non-immi B from Singapore last year, it was straightforward and I had no problems. A couple of weeks later, when I was in Bangkok, I got my WP and the 12 month extension.

The stay can only be extended (not the visa) using a TM.7 (1,900 baht) and only if you meet the requirements - income for your nationality and company paperwork plus work permit. This does not allow you to leave - any travel will require a re-entry permit at 1,000 baht single or 3,800 baht for multi entry.