I would recommend going to http://www.storagereview.com
and doing some comparisons of different disk subsystem tests. Other quality web sites (like TR and AcesHardware) also qualify.
Generally, SCSI is much more powerful at random data being thrown at it. It can handle a lot of small multi-tasked work much more efficiently than IDE can.
If you're talking large monolithic, mostly sequential files, then the only real differential is the speed of the interface and the speed of the drive.
The Seagate 15k.3 is the fastest drive in the world, bar none. An U320 controller on a 133MHz/64-bit PCI bus is killer too, but one thing to keep in mind: these are both WAY faster than a single drive, and you will only use a small fraction of that bandwidth.
Very high performance IDE drives in a RAID-0 configuration will give very good large-file sequential performance. Will they give as good as the Seagate 15k.3? Well check out the StorageReview 15k.3 review and extrapolate from there.
Personally, if you go IDE RAID I'd recommend the 3ware 7500-4 or 8500-4 over any other IDE RAID card. I believe it is the most powerful IDE card (most SCSI-like), and doesn't really reach its true potential until pushed hard.
My guess is that for you, the IDE RAID setup might be just as good as the SCSI setup for reads/writes of big files. If you end up doing a lot of random reads/writes, the SCSI may pull ahead.
Now, another important issue. IDE drives are disposable. Seriously, they die much sooner and in larger quantities, if you analyze a large enough data set. They are not made as well, to get the cost down and the density up. It's just a fact of modern manufacturing and market demand.
Couple that with RAID-0 which SIGNIFICANTLY increases your exposure to drive failures, and you should just plan ahead for the eventual loss you will exprience. Keep decent backups and maybe a spare drive on hand. Alternately, you might investigate doing RAID-10, which gives very high performance and very good fault tolerance.
On the other hand, SCSI drives are much more reliable, especially over an extended amount of time. Nobody can guarantee a drive won't fail, and SCSI drives do fail as well. But the chances are lower, and the drive will likely withstand harder use and still last a lot longer.
For what it's worth...