Wait a second...
If you turn v-sync off you will need to take extra steps to ensure you don't accidentally burn out the player's video card or CPU, both of which have happened with badly-developed games.
What kind of hardware was that? The hardware should slow down in the worst case (due to running out of processing time) o_O That looks more like faulty hardware or hardware not running under the proper conditions or something like that.
Back on-topic: it depends. A fixed framestep is much easier to handle, but you can't go beyond its speed and if it slows down you need to start applying frameskip. A variable framestep is supposed to be able any framerate but it's much harder to handle, especially if you want to handle situations that happen in the middle of two frames that would affect what happens later (e.g. collision of two objects).