I'd probablys say the Zen M Vision.
The iPod looks sexier IMO, but just because something looks pretty doesn't mean it's good inside and out. The Zen can read ogg, Xvid. divx and other video formats that the iPod can't read.
The Zen has its own converter so even if you don't have the right format, it can convert it for you (it's not that long to convert anyways.. like 600 MB takes about.. 10 minutes?.. more or less).
The button was kind of tricky the first time I tested it out, but I got use to it.
I don't think iPods have FM transmitter connected to the player, but at least the Zen does .. for all your.. traffic/weather/lovely music needs....
There's also a lack of accessories for the Zen M Vision, but it's all good, you only need to buy a case and an adapter - I wouldn't think you need any glittery bling case for a Zen.
I think I would only want the iPod for more music listening (iPod nanos) and these tokidoki cases. Even for music listening, I'm sure you'll get bored someplace so having a video player can be a good thing. I mean between the 30 gb ipod and 30 gb Zen, both with video feature, the Zen would be good.
You should buy one for $174.99 (after rebates) at
buy.com or wait for a deal someplace... like newegg or amazon..