This really comes down to personal preference. If you like simplicity, light weight and intrinsic waterproofness, then you might prefer the Ortlieb Plus Roller panniers (the cordura Plus is lighter than the PVC Classic). If you prefer zippers, built-in compartmentalization and accessibility, and don't mind a little extra weight and complexity of design, or being required to use rain covers, then panel loading panniers may do fine for you. I've used both - I've got an older set of the Cannondales, but I've mainly used Ortlieb rollers for years, and recently I also obtained a set of the Arkel GT series panniers. My initial reaction to the Arkels has been that they are rather heavy, extremely well built, and surprisingly I find myself not really taking to the zippered compartments as much as I thought I might. Also, I am not all that impressed with their new Cam-Lock system on the hooks - the rotation of the cams seems to necessitate that the hooks be positioned some distance away from the vertical struts on the rack, otherwise they will interfere. I also find myself actually preferring the relative simplicity of the "100% waterproof" aspect of the Ortliebs - this despite my really agreeing with the sentiment in Arkel's article on waterproofness in panniers:

http://www.arkel-od.com/tips/waterproof.asp

I agree with everything they are saying here, but still I find myself preferring the simplicity of the Ortliebs. They are lighter, there are no zips to go wrong (not that those monster YKK zips on the Arkels are ever likely to go bad), and I find myself a little confused by all the different pockets and compartments. I like simple, and so I believe I'll probably be getting a bunch of silnylon stuff sacks for ordering stuff inside the Ortliebs. The silnylon stuffsacks have a number of benefits - they allow you to keep your stuff readily accessible, especially if you get different colored ones and/or label them with a marker, and they keep your stuff dry even if the bag becomes compromised, and they are very slippery, which makes them easy to slide in and out of the bag. And they are very lightweight too.

To me, it's very nice riding along and seeing a huge storm up ahead and knowing that all your stuff is 100%, completely and utterly waterproof. Some people don't mind, but I find it a bit of a hassle to have to stop and get off the bike to put on rain covers. On the other hand, rain covers do protect the panniers from dirt and mud.

A small (and probably completely irrelevant) aside: I have had shimmy problems on all my touring bikes, until the recent test ride I took on a Co-Motion Americano - and I did that ride with the Arkels, fully loaded. Now I am 99% sure that the bike didn't shimmy because it is built like a tank, but some tiny part of me wonders if there was something about the Ortliebs that contributed to my shimmy issue on those other bikes (the Ortlieb panniers were a common factor between all of them, up until now). I have seen some mention bag sway with the Ortliebs as contributing to shimmy, presumably due to the fact that they are basically just big drybags attached to a back plate, but this seems rather unlikely to me. I'm just holding it out as being possible. However if I get my new Americano and find that I get shimmy with the Ortliebs and not with the Arkels, well then Arkel it is!

We tend to make our intuitive decision first, and then focus on those aspects that support and agree with our position, and ignore (or discount as irrelevant) those aspects that disagree with us. It's natural and human nature. The truth is that either pannier system works well. I would be happy enough touring anywhere in the world with either my Arkels or the Ortliebs. My wife absolutely loves the Arkels, so they are hers (for now)! Me, I'll probably stick with my Ortliebs and be happy as a clam, unless they do turn out to actually be contributing to shimmy.

On the availability of spares for Ortliebs, well, spare hooks are very easy and lightweight to carry. You can also make sure you have spare bolts, and install a second set of top and bottom hooks if you want more stability and reliability. There are also patch kits for if the bags get torn or punctured. Puncturing or damaging these bags is really a very unusual occurrence, not something you see every day.

My approach is to plan for disaster, but not base major gear decisions solely on "what if" thinking - I look for stuff that will be good to use for the 99% of the time I'll be using it, and then just try to take whatever spares and tools make sense to be able to deal with those rare occasions when something does break down. Then I'm happy for 99% of the time and prepared for the 1%, rather than using something that is less convenient for 99% of the time, out of fear for that once-in-a-blue-moon occurrence.

Neil