Speaker power is one of the most misunderstood details in any hi-fi or music system.
Many people are under the misapprehension the more watts means louder (it doesn't) or more watts means better quality (it doesn't).
The problem lies in the fact that most companies who make cheap, basic systems or speakers want to make them appear "better" or "louder" than they really are, so they grossly exaggerate the power outputs. Customers like big numbers, even if they don't really mean anything.
The industry standard figure is " X Watts R.M.S. at 1% T.H.D."
T.H.D. is 'total harmonic distortion', or the figure at which you can hear the sound beginning to break up.
Many companies will use P.M.P.O. (peak music power output), M.P.O. (music power output) or any other meaningless figure, just because people will believe that more watts means it's better (it isn't). These "fake" power outputs can be as much as 16 times higher than the genuine figure.
A good quality 30w amplifier hooked up to a decent pair of high efficiency horn-loaded speakers can be as loud as a poor quality 100w amplifier hooked up to a pair of inefficient speakers.
In the end, just buy what sounds best to you - for iPod speakers, the power output really means nothing at all.