Oracle of Seasons is a sort of tribute to the original Legend of Zelda. It focuses on combat with lots of enemies, and features new (completely new) versions of many of the the old bosses. It still has puzzles, just not as many and not as hard.
Oracle of Ages is the opposite. It focuses on puzzles, and they can become quite complex. There aren't as many enemies and many of the bosses and mini bosses require some thinking to figure out how to damage them.
Which you play first in entirely up to the individual. Puzzles or fighting, which do you want more of?
