Only trouble with the IDE is it maxes about 500k a second. The SCSI will get you up 5MB/s.
For PIO, it rather depends on your CPU. My old PCMCIA atapi CD Rom used to manage about 1.5MB/s, but it sucked up CPU cycles.
Any kind of DMA driven SCSI device will be better, but the first thing you'd want to put on that would be a hard disk.
