G'day Ken
I used Fireline Crystal in 2lb - 8lb until about 18 months ago, then I changed to Power Pro. There was no reason to change other than to try something different. The Power Pro ties a great knot, lies very well on the spool and is substantially softer than Crystal. That said, I pulled out a reel over Easter that had Crystal on it that must be at least 18 months old, and fished with it for the Easter break without incident.
If there was a noticeable difference between the two then I would say Crystal seems to generate more tip wraps and wind knots than Power Pro. I have been using Power Pro on my spin outfits as well as my baitcasters and I am very happy with it. The bright yellow color also makes it easier for tired eyes to see it!
Cheers
Hodgey