Glossy shows more details, hence sharpness or lack-of-it shows more. also for digital, artificial sharpening shows up more on gloss, looks 'digital'. the colours are vibrant on gloss. Glossy prints VERY hard to handle, can't pass photos around or put onto glass frames (as said above).
Matt prints gives a 'classical' film grain feel to digital photos. Less vibrant colours though, but less prone to reflection. Easy handling. Digitalness and lack-of-sharpness not so obvious.
My choice is matt.