With a question like that, you really should list your system spec and preferences on type of gameplay.
Unity is a very demanding game and takes pretty good hardware.
Most feel AC IV has the better gameplay, but you have to enjoy naval battles to really get into it. Otherwise you probably won't like it. It can take some time upgrading your ship and learning how to master battles at sea to really get the most out of it.
AC IV also has a lot of social events that only work if you interact with others online, and they were often buggy. I don't really know if they've fixed them since, but for months they were broken.
Conversely, Unity offers elaborate buildings, with many ways to infiltrate, and detective missions which you can unlock...