In my opinion there are a couple of pros and cons for both games, so I list them up. The easiest way would be download both demo versions and judge yourself <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

Gothic's pros:
- good atmosphere
- NPC Schedules
- nice quests and story

Gothic's cons:
-very small world
-crappy controls
-high skill level

Sacred's pros:
-very big and detailed world design
-great graphics for an ISO RPG
-much action and much hack 'n' slay

Sacred's cons:
-almost no interaction with NPCs, world and items
-simple quests / simple story
-many many enemies and unlimited spawning