Why you should do the IB
1. It's fun, if you're academically-minded
2. You actually learn things, and pick up genuine skills, rather than just rote-learning the text-book like in A-levels
3. It'll force you to be organised
4. There's more variety, because you're doing six subjects from different fields
5. The syllabuses are better for pretty much every subject - you learn more rigorously
Why you shouldn't do the IB
1. It's much harder
2. It's much more work
3. It will destroy you if you're lazy, and you'll come out with a terrible grade
4. Most Universities don't value it as highly as people say. For top end Universities, your offer will still probably be 7,7,6 at Higher Level; yet for A-levels, most of the higher offers are AAA (A*AA for Cambridge). There's just no way in hell that getting a 7 in the IB equates to an A at A-level; for subjects like Maths/Physics HL, getting a high 5 is probably about the same difficulty as getting an A. Only 6-9% of people get 7s in IB HL subjects, whereas 20-25% do for A-levels. It's really frustrating to know you're going to have to work your socks off to make a really tough offer when A-level candidates are putting their feet up knowing they've got straight As in the bag because their exams are so damn piss-easy. For medium tier Universities, IB offers are probably about the same as A-level offers. Bristol's the only university I know that makes sensible offers - AAA-AAB at A-levels and 33-35 for IB.
Do the IB if you're clever and hard-working; but if your sole motivation is minimal effort for maximum reward, do A-levels.