They are both 5 watt LEDs, not sure if the Reflector's Epistars are any different than the 300. The Reflector does have the veg/bloom switch, the 300 does not. That being said, I veg with the old style 300 and have no complaints compared to Metal Halide.
What about the price difference the 300 is like 74$ and the 48 series is 104$? I'd like to take that 300 and use my other led that 40w flood light, would that work?
I am not sure about your 40w flood, guessing it's something that you picked up at a hardware store. You can use that with the Mars Hydro unit, it sure won't hurt anything. The Mars Hydro 300 is a solid unit for the price if you want a cheap LED to get you growing.