I love these jump boxes. I don't bother with jumper cables anymore and I find myself doing jump starts for other people about 10x to doing one for myself. They are just too handy.
What to look for? Good cable and clamp storage. Same for the air hose. If you're fighting those when stored or pulling them out, you'll never have a good experience. I like the Black and Decker units, but there's so many new designs coming out and old ones retired that it's hard to really point at one and say "THAT ONE."
How does it charge? Dual charging is ideal. If you can plug an extension cord into it without the need for a wall wart, all the better. If it has DC on board charging from a power accessory plug in your vehicle, even nicer. I keep one of mine always plugged into an auxillary circuit to keep it charged and ready at a moment's notice.
USB is nice, but mostly a gimmick. Don't get me wrong, it works, but there are better charging solutions that are smaller, faster, and better.
Most use a small gel cel or motorcycle battery. Don't worry about it dying- they are cheap to replace later.
Good clamps. Good solid easy to work with clamps that grab terminals or metal are important. Weak things that slip will make your life frustrating.
Custom colors and racing graphics. Because we all know adding a Type R sticker to the charger will make it go faster, right?