The REx's cabin heating uses a resistance heater...IOW, 1w from the battery produces 1W of heating. The BEV uses a heat pump, so 1W from the battery can produce up to about 4W of heat, depending on the outside temperature and how efficient it can be. The colder it is, the less efficient it is at pulling heat from outside via the heat pump until, eventually, if it's cold enough, it may end up using the same heating as the REx. So, yes, cabin heating will use a lot more battery power in a REx than in a BEV under normal circumstances.
If you will need to warm up the cabin for a trip, it really does help to do it via the app while connected to an EVSE with setting a departure time (or, do it from the car's menu). This not only warms (or cools in the summer) the cabin, but also conditions the batteries. Both versions use an electric heating system to warm the batteries if set for preconditioning when required.
2011 535i x-drive GT, 2014 i3 BEV