A comparison between the two:
Fiance visa
- 6-month fiance visa, allows you to marry in the UK
- must be resident in the UK for 7 days before giving notice to marry
- must wait at least 28 days after giving notice before you can marry
- gives no access to the NHS
- no work allowed
- costs £1,464
- once you are married, you need to apply for an FLR(M) visa, which is valid 30 months and costs £993 + £500 NHS surcharge
- you cannot work or use the NHS (for free) until you have the FLR(M) visa
Total visa cost for the first 2.5 years
= £1,464 (+£551 for priority processing) + £1,493 (+ £590 to apply in person for a same-day decision)
= between £2,957 and £4,098
Spousal visa
- marry in the US (no waiting time)
- apply for spousal visa, which is valid 33 months
- costs £1464 + £600 NHS surcharge
- you can work immediately
- you can use the NHS for free immediately
Total visa cost for the first 2.5 years
= £1,464 £2,064 (+£551 priority processing)
= between £1,464 and £2,015 between £2,064 and £2,615
Edited because I forgot to add the NHS surcharge to the spousal visa cost