I've got both and from what I can see the Cascadias are a little longer and of course the mud flap. I like the mounting struts more also. One piece verses seperate ones. They call them Cascadias for a reason. It rains almost constantly there.

The safety-release is needed on SKS because the material is not brittle. It can fold on itself without cracking and jam between the tyre and fork.
My QRs have released twice, once after catching a section of barbed wire in my tyre and once with a branch.
The latter accident did fold the fender a bit but I was able to use them for another year (4000miles) before the fold cracked.

The full-length ones are much more sociable than 3/4 length which splatter road muck on riders behind you.
My 10yr old ones feature a plastic mudflap but they dropped it in later versions. You can rivet one made from a detergent bottle material.

I got my SKS Chromoplast P45's (for 35mm 700c tires) at REI. Wheeled in my bike, compared the tires to the fenders, looked at how much clearance I would have had (plenty on the front wheel, and just enough on the rear wheel on my Trek 7000), paid up and walked out with them. Took me about an hour to install because I didn't quite understand what I was doing.

They match my bike like they were made for it!

Planet Bike for functionality. Both SKS and PB are good. I like PB because the adjusters are at the fork, not the fender. It helps reduce toe overlap.

Woodys for style.

I've been using the Gilles Berthoud stainless steel fenders from http://www.peterwhitecycles.com/fenders.asp. They aren't as nearly heavy as they sound and are the toughest things going. At 52 bux a pop, not too expensive. If you are considering metal fenders, I'd go with these over aluminum. The aluminum ones are ever so slightly lighter, but will not last nearly so long as aluminum fatigue fails much earlier and fenders are very prone to this, being very thing and getting a lot of vibration.

My current set is 7 years old and going strong. Plenty of commuting, bouncy gravel towpath/trail riding, etc. Another reason these are better than the SKS is that the front fender is a lot longer and will provide better protection for your chain/crank. If you add a mudflap that goes down to an inch or so above the road surface, you can ride in the rain and get no road gunk at all on your shoes and drive train. This saves a lot of hassle for sure.

If you are gonna leave 'em on, I'll give my recommendation to Honjos. They are aluminum, yet less heavy than plastic, and crafted by a in Japan by a family that mainly makes down spouts. You gotta drill and mount them yourself, and they come in 2 widths. A google for Honjo Fenders should yield many hits!