I did a bit of looking around the web a few years back, and finding out that there are 2 different grades of blades made by Stanley, regular and heavy duty. The heavy duty have a treated blade edge.
The snap off blade knives vary in quality a lot. Not just in hardness but some of the cheap ones I have used don't snap off easily either. Some tradesmen may just make half a dozen cuts, then snap off a new one. With a box of knives for 5 quid, it is affordable. It is also handy if you keep losing your knives on site.
Personally I prefer the fatter knife body on the standard box cutter knife. I find I have more control and get a better cut. If you have a bit of arthritis in your hands like me they are easier to hold.
As pointed out, the key is in the tip of the blade. If it is worn away, your blade is done. Blade life also depends on your cutting surface, if it is a cutting mat or wood the blade will last long; if it is a metal or stone surface it won't last long at all. Obvious when you think about it, but maybe people with metal worktop have not thought about it.