I've been using the Nashbar ATB bags. Each bag has two zippered pockets: one in the lid that allows the whole top flap to hold extra items, and one that goes along one end from top to bottom. The side pocket can easily hold a water bottle with plenty of room to spare, the top one is better with flat items.

I don't have a handlebar bag, so those top pockets on the front panniers become my storage for items I might need during the day. I find it helpful for organizing, but I do understand where one, giant, waterproof bag would have its own benefits.

Nashbar's attachment system is so-so. I don't think I've had a bag tumble off yet, but I have had to stop and put the hooks back in place after a bumpy ride. They hold a lot, maybe too much if you're trying to go light. For the money, I think they're a great deal. I can't comment on the other bags you're looking at, because so far the Nashbar ones do the job for me, and I haven't been inspired to shell out more money on another option. I know there are better panniers out there, but the price difference between "great" and "workable" is more than I'm willing to spend for the amount of time I use them.