Whilst tethering is an option it would be more convenient for the laptop to have its own HSDPA connection. My phone uses up more battery power when I have the hotspot function activated, so I generally don't have it turned on. As for WiFi, when you are on the move, having to connect to different WiFi networks becomes a chore.
When using my iPad I really do like that fact that it automatically connects to the 4G network without me having to do anything. So I would buy a laptop with a SIM as long as it runs OS/X or Linux.