I would say both at the same time.
While you are leveling summoning, plant herbs and store in the bank and wait till you max summoning.
By the time you reach level 99 summoning, you should already have excess herbs for 99 herblore (apart from raw materials).
Buy the raw materials and make potions for 99 herblore, then sell the potions + excess herbs. You might make some profit off herblore [[tk_32]]