Go with the WarpEngine. As already stated, it has SIMM slots to add local CPU memory which greatly increase performance. I had to remove my ZIP chips though to get the board working, but what's 16MBs worth of ZIPs when the WarpEngine will take 64MBs if it has two slots, or 128MBs with four. The two slot version is for the desktop A3000, but I managed to hack my case to get the four slot version installed. If you're not afraid to use a soldering iron, you can also replace the clock crystal to easily boost the WarpEngine up to 40MHz. The A3640 is pretty much stuck at 25MHz, I've tried to boost the speed on two cards but never was able to get one running. The WarpEngine also has a SCSI-2 interface, definitely faster than on-board SCSI. You may need to perform the int2 interrupt line mod though, the A3000 CPU slot was designed to only hold CPUs, not additional accessories such as a SCSI controller; again, not a big deal if you're able to use a soldering iron. The A4000 slot was redesigned to include this.
Good luck!
:-D