The answer to this question largely depends on what you're looking for in a patio heater. If you want a model that can cover a large area, then you'll need one with a high BTU (British Thermal Unit). So, if you're looking for a model that can heat up a large space, then you'll want to look for one with a higher BTU.
On the other hand, if you're just looking for a patio heater that can provide some extra warmth to an already existing fire pit or grill, then you won't need as much power and a lower BTU will suffice.