Outside of resourceless champs and energy, I prefer Meditation. I like not having to invest in mana to much, also helps with early mana problems of some champions.
Granted the damage is nice, but to me ( More spells =/= more damage.)
ltmetal2 votesIn general, supports and utility based champs 
want meditation, and damage based or manaless champs 
(any AD champion as well) want merciless.
The Deckowner1 votesmeditation=veigar passive
DeathBurst1 votesIt depends on the champion too much to give a general answer. I guess it can even depend on the match-up. Have a hard lane where you'll be shoved under your own turret? Take Meditation to spam and farm with your spells. Get an easy match-up? Take Merciless to secure the kills.
