What is the best visa option for volunteering to teach English in Thailand: marriage visa or work permit for volunteer work?

I was just offered a volunteer job to teach conversational English at a government school. 2 days a week. I was asked by the director of my soon to be wife's school, so this is not a flaky thing. 2 other places also want me to do volunteer work for them, one might actually end up as a job, in our early negotiations. So, I will have 2 choices, long stay with marriage to thai or teaching\work permit for volunteer work. I am asking the group which might be the best. I have the 400K for bank transfer, but would rather not as it triggers a taxable event, but if marriage visa is easier, it can be done. I guess I am just looking for advice as to what this group thinks my best option might be. Not fake marriage, either. I won't do anything that even slightly smells of illegal. This will be a duly noted marriage in Thailand. TIA everyone. jb