I fly the PT-40 with a 46FX and it is not a problem. Go with the bigger engine, you will spend more time flying at 3/4 throttle, but you can never have too much power in reserve.

BTW - the PT 40 is a good plane, flys well, but do your self a favour and build it with the less angled wing. I built mine with the higher angle and it can be a problem in cross winds.