The ability to put different SIM cards is actually to do with whether the phone is locked or unlocked rather than the branding. A phone can be unlocked but still branded to one network.
When an Android phone is branded you may find network specific apps or features preinstalled which won't be installed on the unbranded version. You'll also find when updates are released the network that has branded the device needs to approve the update before it is pushed out to the phone. This can often lead to delays getting updates as networks have to test everything before approving it. Unbranded devices get updated straight from the manufacturer so there are no delays.
I hope that all makes sense @SKYMAN513?
