Not very impressed with the mobiblu. It's small but technically pretty average.
I think the choice depends largely on your budget.
- fairly cheap, since they make their own memory
- the 250 has an SD card slot for expansion
- it supports subscription services
- sound quality is great, this is the most important thing to me
- video, the screen is very small but surpisingly decent
- supports FLAC lossless audio codec
- it may support subscription services in the future, but don't rely on it
Personally, I think it's worth paying for the extra quality of the U3, but I wouldn't bother with the 1Mb and go straight for the 2Gb.