I've never found anything wrong with the navigation on the SP250, very intuative (a little less so with the remote, but you get used to it.) Plus, it sports the aforementioned remote and built in battery charger which the MOJO does not. While I'm all for a slim version, how much difference do you really think it will make. The Rio is not terribly heavy (as far as CD players go), and the bulk of the size is in the CD format as is. You really only can make it slimmer, not smaller.
And where is the navigation on the iriver unit? Do I have to use the remote for everything? I do find the fact that the remote's LCD is identical to the SP250s comforting, but will size be sacrificed to squeeze all that into the remote?
If you're looking for small,
Try This.. Up to 256 MB of the LESS EXPENSIVE Smart Media, and all the other features. No remote but at 2inches x 2inches, it practically is the remote.
But as far as CD-based MP3 goes, I'm sticking with the SP250. While it's fairly pricey, I'm glad I spent the extra money. No battery worries, easy navigation, cheap media, free remote and case. My 2 cents.