I have a pair of 300-watt heaters on my 200-gallon system - it's plenty. I would go with your initial thought of 2x 150-watt heaters. You could also go with a pair of 200-watt heaters (more than you need, but no harm - and one might be able to heat the tank on its own if the other fails for whatever reason).
Note that if you go with heaters that have their own internal controllers you need to configure them a bit differently, ie: set the Inkbird to the temperature range you want and the heaters +/-1 F lower and higher. That way the Inkbird will turn the heaters on and off.
