Long story short- Multicraft is meant for shared while McMyAdmin is more leaned toward a VPS.
They both have WHMCS modules so nothing different there.
Multicraft has better support (Within 24 hrs).
McMyAdmin doesn't have a builtin FTP client like Multicraft does.
McMyAdmin has a better looking/more sleek UI.
It depends on what you are looking to sell
Can you link me to that WHMCS McMyAdmin module? We've talked with them before, and their enterprise solution seems to be using TCAdmin to deploy instances of McMyAdmin. TCAdmin has a module.
Other then that McMyAdmin is a good panel, nice and light weight, but only works for ONE server. If your planning on using more than one server Multicraft is the way to go, even if McMyAdmin has a better user interface, functionality out weighs looks in most scenarios.
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One thing not mentioned with McmMyadmin is they do have a in panel permissions plug in which is actually nice, though if you have other permissions set up, it will delete the ones you have. Multicraft was my best of all the panels I tried for minecraft servers. In the end it will come down to personal opinion and what features are more important to you. One MAIN thing I did not like with McMyAdmin is having to use Mono with Linux servers uses up just a tad more resources.