I have used the iriver ifp-700 series and the samsung yp-mt6 series. I personally own and prefer the samsung yp-mt6.
iRiver Pros:
-Better voice recorder up to 192kbps
-triangular shaped, easier to read on desk
-larger display
-higher output power 18mw/ch
-a few more minor features, such as the time display
-comes with armband
iRiver Cons:
-Must use the included software to transfer songs, cannot upload songs back to the computer, OR you may use the ums firmware for the player to act as a removable drive but slower transfer rate and lower quality voice recorder
-larger and i personally dont like the triangular design
-40 hours battery life
Samsung Pros:
-rectangular shaped, very tiny, its barely larger than the "aa" battery
-acts as a removable drive and fast transfer rate, up to 18mbps
-white lcd backlight, really easy to read
-blue joystick backlight, the iriver one doesnt have this one
-42 hours battery life
-srs wow sound effect
Samsung Cons:
-low quality voice recorder, only up to 32kbps wav format
-lower output power 10mw/ch
-no time feature
-no armband