Haven't tried the Speedo, but I'm using the Nu Dolphin since a few months for lap swimming. The 1GB version is around 40 quid. It works very well for me and I tend to do swim more often and longer since I have music.
A few general hints to consider regarding these devices: As you fix the player to the straps of your goggles, navigation is "blind". Also, typically swimming action doesn't allow for using a display or complex navigation. I have a fixed playlist with a couple of hours music. It's all selected tracks, no albums or junk. Except for an occasional skip forward, I switch this on when I get into the pool and off when I get out.
I'm using competitive swimmer goggles that come with a double rubber band. You normally twist the band to adjust the space between the eye pieces, which gives any unneeded cable length a good fix. I never remove the player from my goggles, making this a dedicated swimming-only player.
Considering all the above (dedicate swimming, limited navigation), 1 GB of space is more than plenty. Just shuffle your playlist before loading it onto the player if this is what you need.
The ear buds provided with the Nu keep the water out and I don't see the need for the ear clips the speedo has. In fact, I would be worried that these create too much turbulence.
Never tried podcasts or audio books yet so YMMV.