My friend has the RAID, I've got the pro 2a. My friend can hit 1,000Mhz from a Duron 600, while I can manage 936Mhz (9x104FSB). BUT, he can't get a 104 FSB stable, while I can. And at 936, my pro2a is significantly faster than his 1,000Mhz (10x100 FSB) RAID in 3dmark 2000. Plus the Pro2a is a LOT cheaper, is more stable(anandtech.com), and also has ata100 support like the RAID.
In short, go with the pro2a unless you need the RAID capability for some reason.