To me it depends on when you want to SP farm. If you SP farm when you lvl 16 you will have enough sp to get both masteries. IMO Fire imbue is the best you can get damage wise and so it helps you level fast. Grass walk helps get to the mobs much faster which cuts down leveling time but you dont need it maxed out. Im level 58 with Lightning at level 40 and my fire at 58. I run at 47% and for now thats fine. When I think I am to slow I will level it up more
Hope that helps.