I would say the 14mm is a lot sharper than that 18mm all round. I found the images very crisp. The 18mm I found to be quite soft in the corners. That's probably my only complaint however.
The AF is just as quick. I can't say I noticed a great deal in it.
The 18mm is f2 so this give it a slight advantage. It's also pretty compact which can be nice. I used this image guide from Fixelpix as a guide to what extra you can get in before I took the plunge.
If you like manual focus too, you will definitely like the manual focussing option on the 14mm with the distance scale.
I think the 18mm for casual shooting however is still pretty good to use. The 14mm just gives you that little bit extra especially for landscapes etc which is one of the reasons I went for it.
I picked one up from the refurb store along with the 10% offer over Xmas so it didn't come in too bad. No complaints at all with Fuji's refurb either and mega quick delivery.
You would get around £200 to £230 I think if you were to sell the 18mm to put towards a 14mm. A lot of people seem to sell the 18mm and get the 18 - 55. The general consensus seems to be that the 18mm range on the 18 - 55 gives better performance compared to the 18mm prime along with the advantage of a zoom. I have read quite a bit on this though and how noticeable it is without pixel peeping I am not sure.
Hope that kinda makes sense? I don't necessarily want to write the 18mm prime off. Some people criticise it but I think the trouble is that Fuji have brought out some really stellar lenses since releasing the X system so the 18mm can easily be overshadowed when compared.