I'd echo what these guys say. We've taken quite a few elk using tree stands here in MT, and they are very effective....in fact our success rates with them far exceed more "traditional" styles of hunting elk.
Climbers for us here in MT are'nt really good choices, and we prefer hang-on type, like the Screaming Eagle style. I've used a sling style for many years also, and they can work well, especially in trees that may not work for other type stands. They also are great if you have to pack in a ways. But they have their limits too....one being that you will not last as long on stand in one, compared to a more fixed stand. Too much weight on your knees against the tree, and on the steps you need to stand on. Having said that, they will work well for a couple hours. Good luck.