Go for the G5 case. The difference is in Front I/O - only one USB port, but if you're really going to bother modding an entire G5 case to fit an ATX system, there are plenty of aftermarket replacements or upgrades for front peripherals. The G5 fits an mATX system perfectly, even without a motherboard tray. The existing standoffs can be reused and reglued on your motherboard for perfect height on the PCIe slots. Even without a PSU mod, it sits comfortably in the bottom of the case, and the Optical Drive (if you need one) mounts wonderfully. If you're big into modding, you could put the PSU where the hard drives are originally located, and get a hot swap bay mounted to the left of your motherboard. Check out the tonymac forums for guides and stuff, but to straight up answer your question, the premium for Mac Pro cases isn't worth paying - just get a non functioning G5 off your local Craigslist for cheap and strip it out.
