Fiance visa -- pro, you can move to the UK right away-don't have to wait for a wedding, and you have 6 months to get used to living with your fiance before you get married
Cons - you have to pay twice: once for the fiance visa, and once to get your FLR after marriage (I'm not counting ILR after 2 years of after marriage, which you'd have to pay for either way); you can't work on a fiance visa
Spoual visa: pros - You only have to pay once; you can work right away
Con: you can't live with your fiance until after you are married
I chose the fiance visa route because I thought getting used to living with each other (e.g. starting the relationship off on the right foot) was more important than money. It was also useful for us to be living together while we were planning the wedding. And by the time I was able to look for a job, I was already somewhat settled, so I didn't have the stress of looking for work and getting to know my way around at the same time. (I was fortunate to have savings to last me the two months I had in between moving to the UK and marriage.)
But everyone will have made their own choice for their own reasons.