I used to use Hootsuite for scheduling posts on Twitter and Facebook, but have recently switched over to Buffer. Here's why.
Update: I use neither now, and schedule the odd tweet with Tweetdeck. I still like Buffer more than Hootsuite, so I'll keep this up for posterity.
Hootsuite gives you two options when it comes to scheduled posts: regular, pick-time-and-date scheduled posts, and "auto-scheduled" posts. Auto-scheduling works by you giving it the days, time windows, and how many posts per day, and it is supposed to pick the best times for those posts to happen. In my experience, it doesn't work. It seems like Hootsuite just sticks the post in between the two times given, and that's it.
Buffer's method is different. You supply the times upon which to post, and it just takes the next post from your upcoming queue on that time. It's a little less organic than Hootsuite's system is supposed to be, but in reality, it's simpler and better. I frequently change the times my posts go out, and the number of times in a day, just to keep things interesting.
Support for native Twitter photos
Hootsuite doesn't let you post photos to Twitter using Twitter's native functionality, but Buffer does. 'Nuff said. (Hootsuite posts them using their own image-hosting service.)
Better API support
I use IFTTT regularly for a variety of things regarding posting to Twitter. Hootsuite used to give API access to IFTTT, but no longer does. Buffer still allows you to add posts to your queue using IFTTT "recipes", so this makes my life a lot easier.
All that being said, Hootsuite is still a great client for consuming Twitter feeds, as Buffer is only for posting content. The only other downside to Buffer is that you can only have ten posts in your queue on their free plan. Not a big deal for the average Joe, but it does cause hiccups every once in a while.