I'm currently in between the Proto and Snap On Double Box end Wrenches...
You couldn't go wrong with either of them. I have both Proto and Snap-on sets of DBE, and I really can't say which I like best. All of them I have are older, so not a direct comparison to the full polish Proto referenced in the original post. The Snap-on are also late 60's or early 70's, so not the same as today.
I have three different sets of Proto. The long pattern deep offset box, then the ones that are barely offset but still long pattern, and then the lightly offset short pattern. I like them all, but prefer the light offset to the deep offset.
I have the short and long light offset in Snap-on. They're about medium between the two offsets of Proto. I don't think I quite like them as well as the Proto light offset. Again, I think that's a preference for the straighter wrench.
To be absolutely honest, I think the thing I like most about the DBE is the length of the long ones. That, and the handle shape, allow you to turn stubborn fasteners with less pressure on your hands. That's why I also prefer the almost straight ones to the deep offsets. The deep offset ones twist.
I also like the shape of the short proto ones. They are particularly nice if you put them in your pocket, they are nice and smooth, unlike a combination wrench. They are really handy for being light and small. They are very thin also at the head, for tight clearance areas.
I'd suggest trying both types if you can find someone that has them, and see which you prefer from the standpoint of how and what you would use them for. I think that would make much more difference than the quality or the cost.