I'd say go with the 150 watt for both, to be safe. Better to have them heat better than needed, so they can be off some of the time, than have bulbs that need to constantly be on in order to keep the correct temperature. The thermostat will control the temperature after you get it set up, which isn't too hard but can take a little fiddling, so it's a good idea to set everything up and make sure it's heating to the right temperature a few days before bringing your hedgehog home. The thermostat itself doesn't have specific temperatures, it will have a dial with a color range, with red being the warmest. You'll have to use your thermometer to monitor the temperature of the cage, set the thermostat somewhere in the medium-warm range, and start from there. If you come back in a few hours and the temperature is too hot or too cold, you adjust it and wait again, and keep adjusting until it's set correctly. After that you won't have to fiddle with the thermostat again, and the thermostat will keep the temperature in about a 2 degree range where it should be, by turning the CHEs on and off accordingly. So, as long as you set the thermostat correctly, it doesn't matter if you have more wattage than you need - but if you have less than you need the cage won't heat as much as it needs to.
A single CHE is generally capable of heating an area about 5-6 degrees above the temperature of the rest of the room. So as long as the room is normally around 68 degrees or higher, they work very well, but if the room is especially large or drafty and doesn't retain heat well (which is especially true of large rooms during the fall/winter), you might have to put the cage in a smaller room and/or use a small space heater to keep the room temperature at around 68-70 degrees. Two CHEs will work together and possibly can raise it a little more than 5-6 degrees if necessary, but the reason you'll need two is because the cage is so large. If you were to only use one, the middle of the cage (or below wherever you placed it) would be the right temperature but either end (the areas further away from the CHE) would be lower, as much as 5 degrees or so, and you don't want that much variation, even if all the temperatures fall in the correct temperature range.
The best temperature for an individual hedgehog will vary. For instance, if you get your hedgie from a breeder who keeps their hedgehog room on the warm side, closer to 80, your hedgehog will be used to the higher 70s. 72-80 is the overall range; I keep my hedgehog room at around 72-74. With CHEs I'd say aim for around 75, which means the CHEs and thermostat will keep the cage around 74-76 at all times.
Hope that all makes sense. Let me know if anything is still unclear.