How to calculate what wattage transformer to use for LED light strip
Yes, you can use a 40w transformer to power your 37.5w load. However, given the quality of these small power supplies, running at 90%+ all the time will cause heat and premature failure probably. 

You will need to drive the LEDs at 12v. Running them at 15-18v (or anything over 12!) is a bad idea for the life of the LED.  You will need to put a resistor on an 18v supply to bring it down.  I'd recommend simply getting the 150w 12v supply.  Wattage/amperage is different than volts, where you can use a higher powered power supply and the connected lights will only draw what they need (example: running a 12v 50w light bulb from a 12v power supply rated at 150w is fine. Running a 6v 50w bulb on the same power supply is not fine).