I'm biassed because I've been to Meteora and not to Santorini! Definitely worth doing in March without the mass crowds and buses. You could overnight in Kalambaka, but you will need transport to get up close and personal with the monasteries as they are situated out of town in beautiful countryside. The monasteries ooze spirituality and calm and I really think that you couldn't equal this on Santorini. Certainly different and VERY Greek.