First of all boyfriend MUST have visited you, or most visa's fail.
Then as you seem to perfectly know that you intend to marry, the correct visa is FIANCEE, the ECO are NOT STUPID (maybe open for debate with some members

).
Vistor visa gives you 3 months.....Fiancee 6 months....why would you go for the shorter one you are likely to fail with rather than the longer one you are likely to get?
The reason why the Fiancee visa is for 6 months is to allow you plenty of time for a 'TRIAL' of both loverboy & country. The Visitor visa is generally setup for 'family', and those are the ones which actually get them in the majority of cases.