How much warmer then room temp you want to raise tank is the key.
According to 'Aqueon Pro' heater guidelines(ummm ummm),
a 200 watt heater is capable of raising a 90g tank 5 degrees above an average room temp of 68-72.
So with a 74 degree room a 200 watt should be fine if you are not looking for the tank to be in the 80's.
For GBR you want to be able to get into mid 80's easy....
Go 250 at least IMO
Many use 'multi 'set ups for safety and/or even use external temp controllers..
Looking forward to seeing the 120!