Hundreds or thousands of posts in a structured channel?
We've seen Craft installs running with several hundred thousands of users / assets / entries, etc.

But when you're getting into that range, there are several other things to take into account such as hardware resources and architectural designs, various caching methods (Varnish, memcached, Craft's template caching), etc.

Having said that, it makes more sense to me to have them as entries and if you need the ability to have them sortable, then a structured section is the ways to go.

But there is a known issue with the UI for structured sections in the control panel where it will bog down when you get into the thousands of entries range.  We're addressing that for the upcoming Craft 2.2 release, so it shouldn't be a problem for long.