as a semi-regular mid player, i take it depending on the champ or match up. never take meditation on manless champs, but also don't take it if you're building tons of mana or mana regen elsewhere to the point it's negligible early game.
the balance i tend to find is that merciless will help low mana-cost or mana building champs (annie for example with RoA) and meditation helps sustained or repeat damage champs, regardless of mana costs (brand, lux)
however, merciless also amplifies for example brand's burn, so it's good for kill securing.
basically it's preference depending on who you're playing and what you're playing against (I take meditation on alot of people simply so i can spam a little easier)