The 150 is a newer version of the 250, with a new version of the encoder chip that costs less to make. Hence the lower price. Quality-wise, they seem to perform almost identically. The 350 is a 250 with a built-in hardware decoder, which allows you to play back the recorded stream with almost no CPU usage. However, the hardware decoding doesn't work with things like MCE or most DVR software.
The 150MCE is just a 150 that doesn't include any recording software (it is designed for use with Windows XP MCE, which includes its own software). It works just fine in regular WinXP, but you'll need to provide recording software.