My wife and I are planning a 4-day ride on the Katy Trail in Missouri and I need to buy some panniers. I don't expect we'll use these often, maybe once or twice a year or the occassional overnight weekend trip. I'm looking for something that's more than one big cavity per side. It need not be too large for our short credit card touring trips. From backpacking experience, I plan to pack in zip lock bags so waterproof panniers aren't necessary. I already have a standard basic rack. My budget is "under $75" and I've narrowed the field to three that get good reviews:

Avenir Excursion Large - I read one comment that some of the outside pockets don't have any closures to keep things in.

Avenir Excursion Small - I read mention of six outside pockets but I only see two. Are the others tucked away such that they're more secure?

Axiom Seymour LX - Only two outside pockets but they might be made better.

Does anyone have any specific experience with these bags that would make one stand out as much better or worse than the others?

For the record, I've also looked at:

Nashbar ATB - Cheap with good reviews but I can't tell how they open and close from the pictures and descriptions.
Banjo Brothers Saddlebags - No outside pockets.
Bushwhacker Moab - No outside pockets.
Louis Garneau Beta BB-46 - Rather large at 2807 cubic inches.
Sunlite C-Sport Traveler 2 - Not very flattering reviews.