The primary difference is that Mobile Accounts doesn't keep past versions of your system state and user files, like Time Machine does.
What the "Mobile Accounts" feature does is (basically) to sycnhronize the files in your users home directory with the networked version of it during logon and logoff, thus allowing you to have networked home directories and still allow for disconnected operations. This is primarily used for professional environments where people should have centrally managed machines and still be able to take the system home or on the road without having to worry if they have the files they need from the network.
Of course Mobile Accounts still can be backed up to external HD's, even with Time Machine. In fact, I would highly recommend this to make it easy to restore a system in case of hardware damage or other problems.
I think introducing central auth and networked home folders is overkill in a home network, so you will be best off with Time Machine, but if you want this and have MacBooks, you can combine them. Beware though that every sync solution will have a hard time if you change the same file on another computer on the network and a disconnected mobile account and then try to reconnect it. As I have not much experience with Mobile Accounts, I don't know how the OS handles this case.
