you must enter the uk within 28 days of the start date on a spouse visa or you might have to apply for FLR to make your stay upto 2yrs in the uk, and that will cost you money, time and worry. so don't take longer
as for fiancee visa i don't think you do, but i'm not 100% sure, as once wed, you will be applying for FLR for 2 years anyway, so you will have been in the uk 2 yrs, so you can then apply for ILR. maybe someone whos been on a fiancee visa can confirm
