Do you mean something along the lines of calfskin? That's usually glued over a heavier leather, since calf is pretty thin. Some saddles hold up really well, and some not so much. One thing to remember about covered leather saddles is NO OIL WHATSOEVER. Oil can dissolve the glue and delaminate the leather. You shouldn't use straight oil on *any* leather, honestly - modern leathers don't need it, and over-oiling will weaken the collagen fibers and the leather will stretch. If you've ever seen leather that's as floppy as cloth, you've seen something that's been oiled to death. Modern leathers are better off with a commercial conditioner that will contain a good balance of fats, oils and waxes. If you have any doubts, use whatever the saddle company recommends.
All things being equal, I would always choose a non-covered/non-doubled grain leather for durability.
That said, people do seem to like of "covered leather" because it tends to have a tacky, sticky feel. As Kitt said, some covered leathers hold up better than others. Indeed, the term can cover a whole lot of ground, and it can actually cover several different kinds of leathers laid on top of a backing: calfskin, wet pull leather, bull leather, just a super-thin layer of grain leather, weird frankenleathers like the calf leather that's been eco-tanned to have a grained buffalo-leather-like texture, etc. And no, it's unfortunately not as simple as "more expensive covered-leather saddles are always more durable than cheaper covered-leather saddles"--although predictably, once you sink below about MSRP $1800, it's awfully hard to build a durable covered leather saddle.
I happen to own two covered leather saddles, but it's purely a coincidence. I got both of them at fantastic prices, prices that were good enough that I was willing to overlook the covered leather bit.
wow! good things to know. the oiling part had never occurred to me - thanks Kitt! i think i'll end up with the smooth leather as i figured it would be more durable regardless. the look of the covered leather i do like more though - i think it's the "pretty" stitching, which of course is NOT a reason to buy a saddle. haha