Steam - 30% cut
TaleWorlds Store - 100% cut
Steam is the official launcher for Mount&Blade, so you will recieve a key to redeem on Steam, this has been confirmed by Callum.
Callum (Community Manager) - "Yes, the keys we sell on the TaleWorlds store will be Steam keys. "
If you want to support better the game developers you should buy the game on
TaleWorlds Entertainment (Website).