No they are roughly the same. Also, keeping them seperate will save you some money in the long run.
Change your carbon (coal-like) once a month. It gets used-up pretty quick.
Your sponge should be cleaned every week (along with your water changes). Just squeeze out the loose stuff in the sponge using the old tank water (no fresh water or tap water) to remove the gunk but do not sanitize it because it contains helpful bacteria necessary for processing fish waste.
You only need to change your sponge every 3 months or so, depending on how dirty it gets. Sponges provide mechanical (particle) filtration but also a good deal of biological filtration.