I really like my Tungsten|T3 for the expanded screen... it really makes it easier for reading and looking at diagrams, etc.
I would probably avoid purchasing PDAs from eBay unless there is still a warranty... there is just too much that goes wrong with these things to trust one that is not brand-new (unless you really just want to pick one up cheap).
I guess you need to figure out what you want/need in your PDA in order for you to get the most out of it and be content with your purchase:
1. Do you want Wi-Fi (is there a "hotspot" where you can get wireless LAN access)?
If so, you may really want to invest in the Tungsten|C.
2. Do you want to be able to play MP3s?
Most of the newer Palms and Clies have MP3 capabilities, and PocketPCs will have MediaPlayer.
3. Do you want/need a camera?
Several of the Clie models and the Zire 71 all have built-in digital cameras (not great quality, but certainly usable).
4. Do you want/need a phone integrated into your PDA?
The Treo 600 is a combo phone/PDA.
5. Do you want/need a built-in keyboard?
The top Clies and the Tungsten|C have keyboards.
6. Do you want something that is good for playing games?
The Tapwave Zodiac is designed for hardcore gaming as well.
Feel free to visit my Yahoo! Group for additional PDA info, downloads, and links:
IUSMPDA