What is the Best Visa to UK? General or Tousist?

My Russian Girl Friend is looking to visit me here in the UK. I have been advised by a friend to send her an invitation letter. I can support the financial requirements laid down by the UK and will inform visa control she will stay with me at my home for a week. In the invitation letter do I say how we actually met, such as a dating site? Should I tell the truth from start to finish. I have emails ready to support her application showing our meeting in Sharm this year along with photos. My girlfriend says she doesn't want our private messages being read by strangers. She would rather opt for a tourist visa via a travel agent which she feels will be easier to come by. However, I will have to send her a large amount of money as she says the agency insists on payments upfront. I informed her that the uk visa states 'dont book flights or accommodation before your visa is approve' I would prefer the invitation route because, should we wish to marry, the authorities already knew whay she came to the uk in the first place! Please help