Definitely spend the extra few bucks and get a steel-framed gun.
Not only do they reportedly last longer, I've found that in most instances the steel-framed guns are simply better made; their fit and finish is better.
My own suspicion is that brass-framed guns are made quickly, and not as well made or finished, in the hope that the gleaming brass will catch the ignorant buyer's eye and he won't notice or care about these failings.
When I say "ignorant" I mean one who does not know, not one who is stupid. I am ignorant about Russian ballet and raising rabbits, as well as a host of other topics.
There was a time when brass and steel-framed guns of abysmal quality were commonplace. Today, generally speaking, steel-framed guns are better made but few of their brass-framed counterparts have caught up as well.
There are exceptions. I've seen a few well made, brass-framed revolvers over the years. Nonetheless, the brass-framed guns have to be loaded a little weaker than the steel-framed versions, if they are to last for years.
But a brass-framed gun can be a good first gun, if money is tight or if you're not sure you'll pursue cap and ball revolvers as a hobby. They can be excellent teachers; just don't use maximum charges if you want it to last.