Both of them have merits and drawbacks. I shall compare and contrast the two.
The iPod Nano vs. the iPod Video
First, the obvious difference is the amount of storage space: 4 vs. 30 gigs. Now the video part of the iPod video doesn't really matter to me so much because it can only play video specifically formatted for the iPod. All the video I have is encoded with the either the Divx;) or Xvid codecs, and I'll probably never buy any video from iTunes. The extent of the video playing ability that I'll probably use is playing video podcasts and enjoying the size of the screen for browsing the directories. On the flip side, 4 gigs is quite a lot of space for music. If you consider that music encoded at a reasonable bitrate (read: way fucking higher than 128 kbps), you can still fit at least 40 albums in 4 gigs of space.
Second, the size is vastly different. The Nano is tiny -- about as long as a credit card is wide, not much thicker, and less heft than a pocket full of change. The Video has a harddrive. Because of that, it has to have a bigger battery. Because of that, it has to have a larger physical housing to contain all that goodness.
The real argument, though, is whether to get an iPod at all. In my opinion, there are two main drawbacks to iPods in general:
1. Lack of expandability. There is no way to upload a new codec to an iPod. I can't make it play Ogg Vorbis or Xvid. This is a pretty serious drawback because even though MP3 has been a standard for music for several years, it's not the end. Just like everything gets replaced (vinyl, cassette tapes, and CDs), so shall MP3 eventually be surpassed by some better format, and your old trusty iPod won't be able to play it.
2. No lossless format. In iTunes, you can't buy a losslessly-encoded version of a song. If Apple hopes to have downloaded music replace physical media, they are going to have to start offering content of the same quality that you can get on a CD. The iPods say in their spec sheets that they can play something called "Apple Lossless" format, but I've never seen it. This could be due to my ignorance of Apple technology in general, but I've bought a song or two from iTunes, and I've never been presented with the option to download a lossless copy of the song.
Those are my thoughts on the Nano vs. Video iPod. What do you guys think I should get, and why?