I think the supporters' view of holding the semi final and final both at Wembley depends to a certain extent upon their confidence in victory at the semi. If you set off to the semi assuming that defeat is inevitable then it makes the Wembley experience just that bit more special. On the other hand, if you go thinking that a repeat visit is set in stone then you may well think that the first visit is somehow devalued and a bit of an expensive distraction.
I'm sure that most City supporters will feel that they are hot favourites and would therefore prepare the semi to be at one of the big grounds a bit nearer home. At the same time, buoyed by their excellent run of results, I'm also sure that Blades fans will also consider that they have a decent chance to make it through to the final.
Personally I'd prefer the semi to be held either in Old Trafford or even St. James Park (not Exeter!) - both offer the prospect of a big capacity, relatively short journey and a great weekend away.