The Palm T|X is a very nice and new PDA with a great set of features for the price. I own one and love it, and yes, there are quite a huge number of Palm OS apps available. Pocket PCs have a good selection as well though, so I wouldn't say Palm has a huge advantage when it comes to high quality 3rd party apps, only a moderate advantage .
Both operating systems have their strong points. Palm OS is very stable and easy to learn and use. Pocket PC (Windows Mobile) is a bit more powerful and modern, especially when it comes to networking. However, if you like the PDA and does what you need, that's what counts!
There is no such unit as the Dell i30... so I'm not sure what the seller was offering. Perhaps the even older Dell Axim X3i?