I have a DS AR on order from Pete. I've been looking around for some cheap ammo deals and watching the threads here. I get the impression from those threads that steel is less desirable than brass. Is this correct? If so why?

I like the steel cased Wolf myself. Never had a problem with it and its cheap and plentiful.

brass is generally reloadable. steel isn't.

i would not bulk up on steel cased ammo until you have had a chance to test it.
Some ARs just don't like it. I have not had any issues with my unline DS barrel, but I have seen some chrome lined ARs not like it.

Steel case, boxer primed ammo is reloadable. Just not as long lasting as Brass.

Steel cased ammo is generally cheaper, and of a lower quality. The most common brand of steel cased ammo is wolf.

Wolf is great plinking ammo, but some complain that it is underpowered compared to better quality ammo. For 99% of non/competitive shooting, it will be fine.

The big difference, operationally, is that steel cased ammo is harder on your extractor. (Not that big a deal, if you are comfortable replacing a sub-$10 part. Just keep a spare on hand.)

Steel Cased ammo also typically leads to a dirty chamber. This is because the steel cannot expand the way brass does, during the firing of the round. Brass swells just a tiny bit and creates a tight seal in the chamber. Steel does not, so some gas/debris can make its way to the chamber walls. If you don't clean your gun, or if you switch from steel cased to brass cased, this may cause a failure to extract. If you take care of your shit, it's not a problem.

Personally, I run the shit out of steel-cased ammo. For what I've saved, I could have replaced whole guns. And I've rarely had an ammo related problem.