I'd go with the 300 series, by far. Its tuner is capable of a much greater range than that on the Zen, and, additionally, the iRiver has an input for an external microphone.
I would like some advice on which one of these will be better for recording in lectures theaters, are they both able to record from the radio because I thought I could tune them into the microphone frequency and how hard are they to navigate, any probs encountered...
Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic