I've had the opportunity over the past few weeks to try both these devices.   Both devices have almost the same specification - the major difference really are the music buttons on the SP5m.

I really do like both devices however I have some positive and negative thoughts.

Positive - Great devices, amazing screen, integrated WiFi works really well.

Negative - I find the Home and back buttons really small. 

Overall the fundamental decision on these devices is if you want the quick access music buttons - then go for the Sp5m.  For emailing/texting - the slightly larger buttons on the Sp5 are easier to use

Ultimately both device are excellent!

SP5 Detailed Spec Sheet

 

SP5m Detailed Spec Sheet