TVease outlines the differences between MythTV and Windows MCE2005 side by side... of course they offer prebuilt MythTV systems so take it with a grain of salt, BUT it's a nifty/helpful little set of comparison screenshots

Below is a side by side comparison of similar screen shots from MythTV and Windows Media Center. I choose the MythTV skin that most closely resembles Windows Media Center, though I personally like the Project Grayhem skin the best. Hopefully this will help you decide which one you want to use. MythTV is free open source software that runs on the free Linux operating system. Using MythTv will considerably reduce your build cost because you won't need to pay for a Windows Media Center license. Also, Linux offers outstanding performance. You can use an older CPU and less memory, further cutting costs.