Is wiring a 240 volt heater from a sub panel different from wiring it to the main panel?
No, as long as you land the ground wire on the ground bus of course -- but that's a caveat when wiring anything to a subpanel.  (I'd run it using /3 NM so that there's a neutral run to the heater in case it's needed for outlets on a baseboard setup or something like that...and in that case, you also have to remember to land the neutral wire on the neutral bus, but again, that's just like any other circuit.)

The hots hook up just like any other 220V circuit.