Between the two, I'd say IS Boxer is the stronger software.
It offers automatic FTL, which is the strongest assist system; you can set this up with other software, but its a bit complicated.
IS Boxer has far superior mouse broadcasting (HKN is similar, but don't think anything else is even close).
Mapped Keys, can let you do things like move the slaves without moving the main, despite everyone having default keybindings.
Repeater Region based healing is very strong.
You can set up Click Bars, which are Mapped Keys activated by clicking from any window.
IS Boxer is a subscription, $36/year, and can be used on 1-5 computers at once.
IS Boxer is updated frequently, and has very strong support via www.isboxer.com and the IRC (linked from the isboxer main page).


Keyclone gets you 1:1 key broadcasting, which is that's all you need is worth considering.
You essentially have the option to pause broadcasting, or to broadcast.
Along with white listing keys (so only certain keys broadcast) or black list keys (so certain keys don't broadcast).
Not nearly as powerful, but KC is a one-time license (per computer) which lasts forever.