If you only have 1 unit of Saurus, then definitely go for the Buffs. The problem with debuffs is that the enemy may (will) have more than 1 unit to worry about, and you won't get off enough spells to cripple them all. If you are only trying to protect 1 unit, then buffing them will protect against all attackers, whether 1 unit or many.
In a more typical game where you are trying to protect and deal with multiple units, then the decision is tougher. I prefer Light/Life (especially Light since you can buff multiple units at once), but the other lores can be just as effective - it all depends on your play style.