IS Boxer has more features than any other option.
Flexibility, if you need it, is power in boxing like most other things.
Especially once you get the basics of how this software works.

Most of the stuff you can do with IS Boxer, you could do with HKN.
But HKN is script based, so you might need to learn a bit more than IS Boxer.
Like IS Boxer, there is phenomenal support for HKN; either is a great choice.

KC might be good enough for your needs, and you already have a license.

There are other options, but those are the forefront.