I bought both the medium and large in the purseket for my speedy 30. I'm having a hard time figuring which one to use. The medium isn't as tall as the large, and only goes around 3 sections of the bag. Plus one section doesn't really lay flat against the side wall. I like the medium because my bag has the fold marks from shipping, and the medium has play room for the bag. It's more squishy but still has a shape. I think it sags just enough to pull the creases out but isn't too saggy.
Now the large purseket, is taller so offers much more support and shape to the bag. It overhangs a side, which I fixed by putting velcro on it so I just velcro it to itself. This size goes all the way around the bag, which I like, and has bigger pockets as well. But it has such a strong support I'm not sure if I like it. If you put the bag on your lap with the medium in, it kinda flows a bit around your leg. With the large not much moves in the bag. I think they both offer great support and are so helpful with the pockets, but I just dunno which one I like better!!! I hope that helps some, if you decide to go with the purseket.