A higher wattage will cause the heater to turn off more quickly after it has turned on, nothing more. Higher or lower wattage really has nothing to do with efficiency, just how quickly it can heat the water. Electrical inefficiencies are usually due to energy lost as heat due to resistance. A heaters sole purpose is to dump energy as heat, so there isn't much loss at all.
If you have an underpowered heater, it'll be on more of the time, and may not ever heat to temperature. If you have low circulation with an overpowered heater, you may have cool spots in the tank as the heater will heat it's immediate surroundings to temp and turn off, without actually heating the whole tank. If a 125 is doing the job, I'd stick with that. It wouldn't hurt to go with 200, but you aren't gaining anything by running more current through it for a shorter amount of time.