Hey Jim.
Here are the big differences. The sidekick is magnum capable which means it will safely shoot up to 150 grains of powder. The huntsman is for only up to 100 grains.
The sidekick has a shorter receiver and can not be fitted with any other barrels. This way it does not fall under the thumb of the batf and is only a muzzleloader.
The sidekick has either a 24 inch or 26 inch barrel and the huntsman only 24.
If you are looking strictly for a muzzleloader and no other accessory barrels, go with the sidekick. If you are looking to add barrels get the huntsman. However if I were looking to do that, I would buy a rifle caliber and have it fit with the huntsman barrel then you have the choices of any centerfire barrels. The huntsman can only be fitted with shotgun barrels.
I hope this helps