It funny how peoples preferences differ isn't it?
I prefer something like a no.24 blade. The angle on this is much more shallow meaning that the knife is stood more upright. I find this easier for doing shells as sometime the size and shape of the shell doesn't allow you to lay the knife down like you have to with some of the other blades. I also find this easier to do curves as all you have to do is twist the knife in between your fingers rather than having to rotate your wrist.
This is just something i've carried on from my graphic design days really.
Also I only use one blade every 2-3 years.
I just keep re-sharpening it.