Actually, there's barely an advantage to ATA/100 over ATA/66. As long as the burst transfer rate for the drive is less than the bandwidth available to it, then you're fine.
That said, that 8MB cache is also a gimick. There isn't a major advantage to having such a large cache to your average home user (or even your above-average home user).
Look at the performance characteristics of the drive and the prices and decide from there.