I really only use the ZV:M as a Napster client these days.
As a DAP the iPod still has an edge over the ZV:M in my opinion for the immediacy of use, the superior screen quality in actual, real-life use (yes, you read that right... I have had a replacement ZV:M and the situation has not changed) and superior controls. Both sound similar in terms of quality so that is not a basis of choice.
As a video player the Vision M has some advantages including genuinely double the video battery life of the 30Gb iPod and extra codec support, but it will depend on where you get most of your videos for whether the extra codec support is actually useful. In my case, attempting to play DVD's and footage from my PVR (the bulk of the material I would want to watch) resulted in the same rather time consuming transcode requirement as the iPod would in the same situation. As a photo player it has the advantage of some extra manipulation features, but it has no USBOTG facility... a curious omission. FM radio is slightly worse than the iPod remote implementation, and voice recording is also pretty mediocre compared to what the 5G is capable of with third-party accessories... but you do get them with the player at least, and not have to pay extra for (better) results.
Overall, it's a pretty evenly matched choice given price/performance but in terms of a DAP the iPod still retains some advantages for me. My choice happens to be the iPod, although I'll be keeping the ZV:M until I find a PlaysforSure player I can get on better with.