the flash they are using is quality stuff so the nano might be a good idea
remember though how flash memory works, it only will write a few thousand times but read near indefinitely. that means that if you change your music you are carrying a lot the hard drive may be better, but if you're a load and listen guy then you probably should consider the nano.
Flash memory has improved quite a bit. Between the design of the memory and wear management logic built into the devices, you can expect to get 250,000 write cycles out of the chips. In an iPod Nano, that would be many, many years of daily use.
Unless you need the disk capacity of the 'Classic' iPods, I'd get the Nano. It's pretty rugged, with the lack of moving parts making it ideal for wear while exercising. (It also works pretty well for wearing while napping.)
Here's my unbiased retired Apple employee overview of the models:
iPod Shuffle - cheap, no display, OK for workout music or a simple audiobook player.
iPod Nano - good basic player, also can show photos, teeny videos, and plays games from the iTunes Store
iPod Touch - iPhone without the phone; Great for photos, videos, pocket Internet access (web, e-mail) over WiFi; Will run iPhone App Store stuff wit a software update coming in July.
iPod Classic - Hard-disk drive iPod; Good sized screen with humongous storage capacity. Put your 10,000 CD collection in your pocket...