I have this one
http://www.amazon.com/Serfas-Speed-B...erfas+seat+bag
The bottom unzips to expand downward to basically double the capacity. I usually carry in it a tube, a CO2 cartridge and inflator, a multitool, and a tire lever. It has an internal side pocket that I slide my ID, insurance card, and credit card into. It also fits my iPhone. There is plenty of left over room. If I expand it I can fit two or three tubes plus he other stuff. It's not water proof but it's resistant.
I rarely carry anything in my Jersey pockets because it's uncomfortable.