Slee_stack's blackburn find on Nashbar looks like a killer value, but it needs to mount on rear rack braze-ons (can't tell from your picture if your bike has them or not). Also, it doesn't specify that it is disc brake compatible. Having fitted 4 disc bikes with rear racks, I can tell you even many of the "disc compatible" racks may not be compatible with YOUR discs. In other words, every bike seems to mount their discs and route their cables in slightly different ways and rack manufacturers are hard pressed to design something that takes every possible permutation into account. Every one of my rack installations required shopping around for a rack that would actually fit my bike and then doing a little dremel work on the rack.
Your amazon rack looks promising as far as disc clearance, however given your bike is aluminum, I would be a little uneasy about clamping the seat-stays like they intend. Steel bends, but Aluminum tends to break/
If you live somewhere with a decent bike shop or two, you may save yourself a lot of headache by shopping in person so you can actually hold all the pieces up against your bike and make sure it will work. Because once you have a decent rear rack, you will be SO glad you have it. It really increases the utility of your bike in a huge way.
As an aside, I'm not a big fan of rear trailers. At least not inexpensive ones. The ones I've tried really make the bike handle awkwardly and they are very big things to store when not in use.
