The differences are probably in the design/size of the reflector and the glass used on the front of the bulb. Either will work, but you want a nice wide beam (PAR37 or greater) and enough wattage to achieve the proper surface temps around the enclosure.
I picked up an Arcadia halogen. It seems very similar to a GE PAR38 but the glass used in front looks/feels closer to the Philips PAR38. They all seem pretty close to eachother in terms of brightness and heat output.