It's pretty much only a visual difference. Both are interfaces that displays all the data from the Steem blockchain. When you make a post from either of these, it gets published on the Steem blockchain, and then both of these two retrieves the information from the blockchain to display it on their website.
So you are free to use the one you like the most. There are a few minor changes though, such as busy.org having the voting slider available to everyone, even for users with less than 500 Steem Power on their account. Another nice benefit to using Busy is the fact that they give you an upvote if you tag the post with "busy" when you create a new post.
Basically they both are Steem applications and they both runs on Steem Blockchain.
While Steemit is kind of more preferable to most of the users because of its popularity among the bloggers but in compared to BUSY.ORG there are some features that really makes busy.org kind f little more special than Steemit.
For instance the User Interface .
Steemit interface it is kind of easy to grasp and there are not that much information found and seems kind of empty in some places (But it is the site that is used by most of us)
While Busy.org it shows much more information compared to Steemit.
You can see your rank as a Plankton/Minnow/Dolphin/Orca/Whale below your username.
You can see how much is your Voting power ?
You can see what your upvote is worth ?
In Steemit you can even use your Posting key to log in and do your usual activities but in Busy.org you will have to use steemconnect ( owner key, active key or master password. ) to do the activities .
While there are two bots of Busy.org who will upvote your content if you use the busy tag in your post. It depends on the SP that you and the people who are following you. All of you and your followers SP combined decides how much your upvote from the bot you will get.
While for users who cannot upvote at percentage basis busy.org (users with less than 500 SP) they can use the Vote Slider and they can give any amount of upvote they want to any user.
They are what you would call a different front end. Imagine if you had 2 identicle shops and you just put a different store front on them. that is basically what it is. just a different front end for the user expierence.
As frontend interfaces, I see minimal differences, other than the fact that busy seems to pay nicely to some users, and completely ignores others that use their frontend regularly, this is also true for other dapps on the blockchain. I still use busy regularly, but it would be nice to receive their votes every now and then.
It is just the front end interface basically. Busy is much more user friendly.