The fiancé/proposed civil partner and the marriage visitor visa are both valid for six months and allow you to get married or enter into a civil partnership in the UK.
However, marriage visitor visas are for individuals who intend on getting married/entering into a civil partnership in the UK but do not want to stay or settle in the UK permanently. After 6 months, marriage visitor visa holders must leave the UK and cannot extend their visa or apply to switch to a visa under the spousal route.
Only a fiancé/proposed civil partner visa allows you to apply for a visa under the spousal route in the UK after you marry or enter into a civil partnership. This may then lead to indefinite leave to remain (“ILR”) in the UK after 5 years.
It is therefore essential that you understand which visa you are applying for.