I'm planning to go to Chiang Mai for about 5 and a half months later this year for the purpose of visiting my boyfriend. Which visa would be better to apply for?

The multi-entry tourist visa is slightly cheaper but would involve 1 more border run than if I have a non-imm O. The O visa application says I need a sponsors letter. Is that basically just a letter from my bf inviting me to come and see him? Could this be in the form of an email or would it have to be handwritten?

In future I'd like him to visit me in the UK- would having had an O visa look good when applying for his UK visa? (in regards to proof of relationship)

Thanks for any help or advice.

A Non Imm O Multi Entry Visa can be easily applied for at the Hull Thai Consulate.

http://www.thaiconsul.co.uk/downloads-and-visas.aspx

You can then come and go as you want for a year, but as you say a maximum of 90 days each stay.

A sponsors letter is not needed but never a bad idea if you have one.

Each entry of a Tourist Visa can be extended at Immigration for a further 30 days 1,900 Baht fee.

Hull will issue multiple entry Non-O visa on basis of visiting friends. Paperwork requirements are minimal: passport, passport photo, visa application form, fee. Turnaround is usually a couple of days. There is no requirement to provide a sponsor letter for this visa. The website is confusing as Non-O visa can be issued for a variety of reasons, and the consulate merges the different requirements for the different reasons for issue together.