Firstly, there aren't a whole lot of dead blow ball peens on the market.
The ones that are on the market are generally USA made, and cost almost what the Snap-on ones do. (example, go to mcmaster.com, and search for "dead blow ball peen")
The Snap-on hammers have precision machined faces, as opposed to hand ground faces. The injection molding quality is in a league of its own, and they have high quality TPR grips whereas most are just bare slippery plastic.
Finally, you can beat them until they literally fall apart, and get a new one for free. Not only are they the best dead blow ball peen on the market (by far), you only have to buy them once in your lifetime, so the price is somewhat irrelevant.