Apple Powerbook recommended?
 

Would it be recommensable to but a Powerbook off of eBay and install Gentoo or Ubuntu on it?
Moneywise (that's why I want to buy it off of eBay)
Powerwise (Apple = Power)

Or is a PC laptop a better idea for linux?

Depends on how old a model of PowerBook it is. Also note that some proprietary Apple hardware don't have (or have poor support by) Linux drivers, and those that do are supported better by certain distros than others. Proprietary Linux software like Sun's Java and Flash often don't have PPC versions. Otherwise, if you can get it working, Apple computers are decent I guess.

An 80x86 or a notebook PC is better because most of the hardware is supported. Stay away from SIS and ATI chipsets. For video graphics chip, look for notebooks with nVidia. For the hard drive pick PATA instead of SATA. A Pentium M is better than Pentium 4 mobile and Pentium Celeron on terms of performance and battery life. You do not have to look at just eBay, I suggest searching some computer builder sites that do not include an OS in their notebook computers. Cyberpower and Hypersonic comes to mind. I recommend at least a 3 year warrenty for notebooks but only if it also covers accidental coverage.

If you bought an Apple server, I would say yes to install Linux because MAC OS X is a poor performer for server environments. I suggest keeping MAC OS X for desktop environments.