Neither - and both. LOL!
You'll need to periodize your training but you'd be amazed at how small of a change can help - something as simple as changing your workout order, your rest intervals or your rep range can do. You can do one workout in straight sets and the next in antagonist pairs. Or vary between unilateral and bilateral work.
No need to completely rebuild every six weeks. Look for p-funk's posts, I believe he wrote something good about this here or perhaps it was on his blog? Also look up Gazhole and cowpimp too - they've both written on periodization here.