I have a 30G Creative Zen Vision M, which is like an iPod, except that you can rip songs from your CD's and they will play on this player. I am not limited to one music service to get music. Like the others mentioned, you can only get DVD's/music at iTunes or download a converter to convert your DVD's. This might not be an issue for some people, but for other people it is. This was enough of a factor for me personally to decide against getting an iPod. If you were to go with a different player, be aware that it is harder to find accessories for anything that is not an iPod and iPod accessories don't work with non-iPod players.
In a lot of these larger capacity players, the battery is built in, so when that battery won't recharge anymore, you have to replace the entire player. Movie files take up more room than music files, too, as well as battery life. I don't watch movies on my player because I don't like watching video on the tiny screen.
You might want to check out other sites, especially online retailers, that sell mp3 players/iPods and read the customer reviews of people who bought them. My DH buys computer parts from Newegg.com, and they sell mp3 players and iPods.
You might not make your purchase online, but it helped me when I was shopping around to read what other people thought of certain players. I ended up buying my Zen at at the store.
A portable DVD player might actually serve you better than an iPod, but it all comes down to personal preference and your own needs. iPods are easier to carry around, but you can play DVD's you already own on a portable DVD player. Most DVD players also play audio CD's, too. Again, you would have to look around to see which player does what and decide on what you think will work best for you.