I currently have two Seagate drives in my older PowerMac G4. Both are 120 GB but one has 2MB cache and one has 8MB. I know that the performance tests show that the 8MB is a better performer, but it isn't something I can actually perceive on a day to day basis.
However, any recent 7200rpm 3.5" drive will likely outperform the drive that came in your PowerBook. I also have an original TiBook that came with a 30GB IBM drive. Performance is downright pathetic compared to the Seagates.
If the prices are similar, I would suggest that you buy the 8MB cache version since it likely will be slightly faster.
Also, I know that most drives only have a 1 year warranty now, but some of the 8MB models still have the old 3 year warranty. This is likely the best reason to get a 8MB model.