Depends really. I've got two 24" Cinema, a 27" Cinema and two 27" TB and use them all. My office is dimmed so gloss isn't an issue, and the glossy screens are beautiful and clear. Way better than any matte. I do software development so maximum real estate is what counts, not PPI. I have the three 27" on my nMP alone.
So sure they're packing in more pixels per inch but I doubt I want them. My text will look a little clearer - so what? I'm working on digital bits here, I don't need to make it look analog. I also have 16 spaces for all my different tasks, how will that perform with a 5k center monitor? Don't know, and not sure I want to find out. What about a 10k - will we ever need that many pixels? 5k is already out of visual acuity.
Further three monitors is ideal for me. I need many windows up simultaneously and those three 27" partition it perfectly. The only thing that might be better would be a continuous wrap around screen.
Anyhow I don't get the iMac at all. If that's all the computer you need, why don't you just use a laptop? If you could use more, then why not get a Mac Pro for a few hundred more? I can see where it makes sense for some use cases (total simplicity one monitor computer), but if you're serious I don't see the point.
