First, of course I will qualify my response by saying I 'work for' iRiver
All of the iRiver flash players can be used as a portable drive, with no drivers necessary, however they all require a USB cable for connectivity (one benefit with the iFP-300 series is that they use a very standard mini-B connector that is compatible with many (Canon, etc.) digital cameras and other small devices.
To be fair, CL is currently shipping a new Muvo device that has the direct USB connection at 256MB, Rio is also shipping a 256MB version sans cable.
It does depend on your use needs for sure, one reason we have stayed away from the direct connection is that it allows us to do two important things:
1) Put a (relatively) large, backlit display on the device
2) Use a standard AA battery
Both of the above can be convenience items (i.e. with 256MB worth of music, it's nice to be able to browse them by title with a nice display and using a common AA, you can exchange your battery easily when travelling, etc.).
As a side note, in the past I have extended a discount to those NSXPrime members that are interested and would continue to do so if you do become interested in an iRiver piece ('PM' me if that is the case).
Good luck with your MP3 hunting