I depends on where your rear caliper is located as to whether or not you need a "disc compatible" rack. If the rear caliper is located between the stays, then any rack should work.
I use a
Tubus Cosmo on my Trek Portland with the rear disc caliper located between the stays. For top quality, you can't beat Tubus. Best pricing is at
TheTouringStore.com
I like two separate panniers instead of two joined together. I like that if I only need one, I only have to take one. It also makes it easier to put them on the rack and take them off again. That's the primary reason I like Ortlieb panniers. I hang my bikes to park them at both ends of my commute. The Ortliebs go on and off one-handed while I hold the bike with the other hand. There's absolutely no fuss with the mounting system.
I too didn't want anything too large, so I use
touring panniers designed for the front, even though I hang them on the rear rack. They're smaller than touring panniers designed for the back. I get four shirts and a pair of jeans in one, the week's worth of lunches in plastic containers, apples, oranges and snacks in the other. With room for a hardcover novel in each (an occupational hazard), my lock and assorted sundries. Ortlieb also makes panniers designed for laptops, but frankly I think regular ones would work just fine.
Conveniently,
TheTouringStore.com also carries Ortleib. They don't have a shopping cart on the site, you have to call and talk with a real human to order. They'll also make sure things you're ordering together will work together and for your application.
EDIT: Hey, it looks like they'll be having a sale on Ortleib bags after Labor Day.