Cash most likely just due to the fact that Rsgp is fun when you have it but you don't necessarily need it to have fun.
Where as you need $$ to have fun in real life.
Really wouldn't make much sense not to take the cash, as you could spend it all on RSGP if you wanted regardless, and it gives you the option to do whatever the fuck else you want with it.
500M gp. I can make cash irl way easier then I could make 500M on RuneScape. To me its easier to pick up a couple shifts at work (i love my job) and earn some money than to goldfarm.