In almost any review that I have read, Sonnet upgrades are slightly faster than others. Powerlogix offers a really nice selection of dual G4 upgrades and they seem to have a great price to performance ratio. My personal recommendation would be the OWC Mercury upgrades or the FastMac upgrades. They appear to be custom brands of the same generic upgrade so you will be getting the same product. I guess you should just compare on price. I have an OWC Mercury upgrade from http://www.macsales.com that I installed on my AGP G4 PowerMac and it was a stressfree problem free experience.

Hope this helps.