Well solely in terms of the costs of Visa Fees......
Although some folks feel that the Fiance(e) Visa is somewhat more tricky to manage in terms of proof of relationship, intention to get married, and intention to live together permanently in UK.
Additionally, under a Fiance(e) Visa, a wedding still needs to take place in UK .
Until the FLR(M) is approved the applicant is prohibited from working. (either paid work or unpaid work such as volunteer work) .......potentially a cost there too......cost involved due to lost opportunity for earnings
As always it's a personal choice.....everyone has their own view on which one is the best for them.
Every application for a visa has it's risks.