I myself always opt for full sized, and in fact, even when I was building myself a decent MediaCentre, even after I spent a tidy sum on a really good MATX setup and a sweet case and everything, I changed my mind after only a few days and I ended up using my old AMD Hex core setup and buying a Zalman HD160.
I think another reason was that I wanted to plonk a number of cards into it, and even right now, it has a Radeon 7770 VapourX, Asus Xonar DG, a dual PCIX TV Card ,and two single PCI TV Cards ( 4 recording channels ) also SSD for C: and 2x3T Drives in RAID for the Media - Waste of money but great fun.