These two bags are presently the best options for portable use. I have both, and find they excel in slightly different area's.
I find the lowepro is best for belt use (with its secure velcro belt loop). Negatives are the secure velcro belt loop (which makes quick screen checks difficult) and the shallow depth (won't accept my ipod with attached MyPower).
The waterfield bag is large enough for ipod, amp, phones, accessories, extra batteries, foamies, (well you get it). Unfortunately, there isn't a good method of securing the bag to your person, so on the go usage requires some ingenuity.
Both are really well made, and have held up quite well to daily abuse. You won't be disapointed with either, despite their minor shortcomings.