From what I have read up on the pumps and the fact I do have one and have tested it out one time to just see how it is, I can tell you that the size does actually matter. You are suppose to measure your width once you have an erection and then find out which one is right for your size. Here is a link to the site that I got mine at, this link is for the chart. The price on the one you have found is better then the site I got mine at so I would stick with buying that one plus mine doesn't have the gage which I have found out is very important.
http://www.newart.com/picksize.html
It fully explains how to pick the right size on there. Beyond that I don't have any more info for you as I am still knew to all this and I won't be really using my pump for about another month. I am just not at that stage yet.