As far as I have seen scanning ebay and other ads there is no noticeable difference in the used price of Mark IV a and b amps. That suggests to me that neither one is particularly more sought after than the other at the moment. IVa's would be rarer which might make a difference for collectors in future but for now I can't see it.
I think that there is a noticeable difference in sound though. My reference for classic Boogie lead tone was a guy I used to see in the early 90's who played a MarkIIb cranked very hard and driven a bit further by a little Korg A5 pedal. There was a really unique vocal 'yowl' to the sound that has really stuck in my head, its difficult to describe but I know it when I hear it. I tried a MkIV in the early 90s that had that sound - but I could afford one at that time. I had a MkIII for many years but couldn't quite find it there (At least not while keeping a good clean sound). Later on over the years I tried several iv's in shops which would have been b's (at that time I didnt know that there were different versions) and was always frustrated because I couldn't quite get back to the sound of that first one I had tried. Finally tried a nice used IVa and there it was.
I can't comment on the IIC+ thing because I've never heard/seen one live in a room but in my experience the IVa lead may be a bit closer to the older Mark II sound and possibly a bit more versatile for pop/blues/rock styles and the b is a bit higher gain and may have been voiced a bit more towards the heavier rock/metal people who started to use them. Its probably a quite subtle difference in reality, so as always I think any preference between them comes down to the reference sound in your head that you are trying to work towards. They are both great amps!!