The Mizuno Samurai Pro Series: GXC31 Catcher's Mitt can be used for softball, but it features a smaller pocket designed for baseball so you will want to consider that before purchasing.
Besides the Mizuno Samurai Pro Series: GXC31 Catcher's Mitt being bigger, the main difference will be in the leather construction. The Mizuno will be easier to break in as it is constructed from full grain leather shell while the other catcher's mitt is comprised of steerhide leather.
I would actually recommend the Mizuno Samurai Pro Series: GXC31 Catcher's Mitt, I think it will ave the necessary padding and it is very much worth the money. If you wanted to look at something a little more high quality the 2016 Mizuno Global Elite Series Catchers Mitt: GXC11 is a great option. Really you cannot go wrong with either one.