The inner bag is the way to go, IMO. A first aid kit attached to your bike is not as useful as one that can be easily moved under duress.
Thats actually a really good point. To date every time I've wanted my med kit, I have had to take it with me from my bike to where I was treating the person. For example with minor scrapes and such, I went with the person to a nearby stream to flush out the wound, then applied bandages. Whatever bag I get I'll either want another bag inside, or for a minimal number of velcro straps to facilitate easy removal.