So, not only 4k pixels on the iMac 21", but also 25% improvement in colour gamut. This screen should look fantastic!
I'm partial toward matte too, just got an Asus 4K display that's seriously impressive, never expected them to brand really nice displays.
Personally I love glossy screens. They get glare sometimes, under direct light, but nothing that an angle change can't fix. Besides I don't want to be using a laptop under direct light anyways -- and for a desktop which is used inside, that's a non issue.
Non glossy screens (which I started with), I now find smudgy and hazy, and with much worse (gray-ish) blacks.
Under controlled