I am unsure about the differences in success rates for fiance versus spousal. Usually deciding which route to take is usually determined by finances. It is usually cheaper for the couple to marry in the US and to have the US half apply for a spousal visa to come to the UK. If you decide to go the fiance visa route, you'll have to apply and pay for the fiance visa. Then once you marry in the UK, you'll need to switch to FLR(M), which is another visa with more costs. And just to clarify, the spousal visa isn't a 'marriage' visa but a married one, if that makes sense. Fiance visa is intended for clearance to marry in the UK and with the later intentions of applying for FLR(M). Spousal is clearance for those already married.
So it all boils down to if you want to marry in USA or in the UK and if you want to apply for one visa or two visas.
If I'm incorrect in any of the above information, I certain one of the much more versed members of this forum will sort it out. Good luck!