Originally Posted by jcalvin
For what it is worth, I read somewhere that the way a ladder gets its rating is.... max extension, laid horizontally on supports at each end, and the respective weight must be supported with minimal deflection in the center of the ladder.
That's what I remember from fire-school.
FWIW, I think the 300# ladder will definitely be more stable, but I'm not sure I'd say it will last longer, necessarily. It will definitely be harder to move around, and with a large ladder this can be significant. Especially if you plan to move it yourself. Stability is a personal issue, to some extent. Some people are comfortable on a ladder that has some wobble to it. Other people don't want to be on anything that feels short of completely rock solid.
Something I'd pay more attention to is the quality of the parts. Specifically, look at the dogs (or pawls) - those latches that hold each rung as the ladder is extended. Avoid ones made from stamped sheet metal at all cost. These should be substantial cast-aluminum assemblies, with stout springs, and smooth action. If that means going up a weight rating, then to me that's far more important than reducing a little wobble.