The thing is that there's a very different intended purpose for the two devices.
While a PDA would allow you to write papers, this would require a separate keyboard for comfort. Also the small screen and limited connectivity of such a device would create problems viewing documents and printing or sharing the information in many conventional ways.
A PDA is really meant to be an accessory to a computer, whether it be laptop or desktop, not to replace either. If the choice is only one of the two, I would recommend a laptop. While it is more cumbersome, relatively speaking, the software is far more powerful and the uses are more varied.
If you really feel the need for a portable datebook or organizer, then looking for an older model Palm (such as the venerable Palm IIIxe or Palm Vx) online would be a good choice, especially starting out. There's always Palm's (PalmOne now) lower prices offerings, the Zire line, if you feel that used is not an option or prefer a more updated option.
If you require more information regarding handheld computing devices,
http://www.brighthand.com and it's forums are an excellent place to get info.