The watt rating of the resistor is the size, the bigger the number the bigger the size and the more heat it can dissipate. The 50w size is marketed for the turn signals. The 25w 6ohms are the same ones you have now, except they're smaller...
The 25 ohms has more resistance; the lowest load resistor I've seen are 3 ohms. I still don't know if those would work for you either.
I was thinking check the wiring for the load resistors too, double check that you didn't wire them to the parking light wires.