Advantages of Statue of Zeus:
- Early availability, producing powerful units good for early war
- Horse-mounted units that don't require Horses or the Horse-riding tech
Disadvantage of Statue of Zeus:
- Requires a resource (specifically, ivory) connected
Advantages of Knights Templar:
- 5/3/1 units, devastating on the offensive, and equivalent to Pikes for defense
- Relatively cheap to build; a switch from Hanging Gardens or Sun Tzu's often results in single digit number of turns to completion
Disadvantage of Knights Templar:
- Comes at approximately the same time as other useful military Wonders--decisions, decisions!
Personally I like both, and to have both means having a potentially massive military advantage, but given a choice, I lean slightly toward SoZ; I have fast units with 3 attack strength in case I need to push my frontier in the early game with 'pointy-stick' settlers.