WD Raptor 150 and Aperture??
First, the Raptor 150GB is not compatible on older G5's (prior to Dual Core), because of their SATA protocol implementation. In cases like that, you can just add a different SATA card (controller).
To answer your last question, I believe the Raptor would be faster via the eSata. However, I don't think it will be that much faster in return for the extra expense. If speed is your concern, I would get at least a 2-bay enclosure and RAID 2 SATA drives together via RAID -0-. This will give you a better performance increase than a single Raptor; unless you want to RAID two Raptors! Keep in mind that with a RAID -0- if one drive fails you lose all info.