I uninstalled the trial of Diskeeper Pro Premier and installed a trial version of PerfectDisk.
The first time I tried to capture a VSH tape via Canopus (as I had been doing while testing the performance of Diskeeper) Pre El 3.0 froze. It was unresponsive at first, and then Pre El just closed by itself!
I tried three more times and the last try worked. I can't say for sure that it was PerfectDisk causing the problem, but this is the first time I've had any problems with Pr El since I installed a larger power supply, a dedicated internal drive for video, and two extra cooling fans for my three internal hard drives.
It may be my imagination, but I think that other applications were running faster with Diskeeper than PerfectDisk.
I've also thought about the option of "selected file defrag" for PerfectDisk-- Why would I need it if it is defragging on the fly, anyway? Diskeeper does have the ability to "defrag only fragmented files" manually.
So I'm now leaning towards Diskeeper Pro Premier, taking into account your recommendations and my recent experience.
I will give PerfectDisk another try by creating and editing the same project I used to test Diskeeper, just to attempt a less than scientific, somewhat objective comparison.