iMac 21.5in plus 24in monitor or iMac 27in

I'm considering an iMac i7 mainly for photo-processing using LR. In most respects the 27in model is a better choice than than the 21.5in. However I don't fancy the glossy screen so I thought of getting the 21.5in model plus a matte 24in monitor (NEC or Eizo). That way I get plenty of screen space but also a matte screen for photo work and hopefully not too much space on my desk taken up with hardware. I did consider the Mac Mini but I don't think it's well-enough specced and not good value for money.

Are there any disadvantages to the approach I'm considering or issues that I should take into account?

TIA.