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bandwidth mostly I would imagin. text uses a great deal less bandwidth than images so ssh would be a great deal more responsive than rdp/vnc.
I'm no expert on rdp/vnc but i'd imagin both should support encyrption - ssh is also encrypted. personally though, i wouldn't trust vnc's security over the internet though. not unless you have a fixed ip and block vnc's listening port to all outside traffic from ip's other than yours.
vnc isn't multi-user. I think rdp can be depending on the server, however as far as i know linux only support hosting vnc (as rdp is a windows protocal). ssh is multi-user. what this means is your mum can be logged in and you can make changes without affecting her while she does what-ever it is mums do on computers.
the only draw-back to ssh is you'd need to know all the command line administration tools - but there's heaps of info about them online so you shouldn't run into too much difficaulty.
going back to security for a moment - even if you choose ssh, it would be a good idea to block outside connections to ip's other than yours (again assuming you have a fixed ip).

i should have added - if using a GUI to admin is a must, then it would be worthwhile looking into X11 tunnelling via SSH rather than VNC.