This was a closer poll than I expected.
I honestly don't like either ability all too much, but I have been swayed to support Magic Guard (sorry Breloom). And the reasoning behind it is simple, it fulfills the concept, despite what all the naysayers suggest, let me explain.
Magic Guard allows further customization of CAP10, and the easiest example is whether to counter your target either offensively or defensively by use of either Life Orb or Leftovers. Lots of arguments in this thread have hinted at the 'awesome' offensive potential of LO + Magic Guard, but it takes a serious toll on CAP10's ability to be defensive w/o Leftovers recovery and defensive EVs (noting the calculations throughout the thread). Yes, it still will be formidably bulky thanks to its HP, but aren't Metagross and Gyarados extremely bulky as well, while being much more powerful? In order to be the 'bulky' sweepers they are though comes at the cost of missing out on those 2HKOs or 1HKOs that they need due to EV allocation, and the same applies for CAP10. Whether or not it's worth it is up to the user (and looking at its offensive stats, I'm not sure I'm a fan).
There is a lot of overestimation on this thing's offensive prowess, and one of the main reasons for this is not taking into account the multiple ways CAP10 can be brought down. Since when have players (besides extremely heavy stall) relied exclusively on SR, spikes, poison to kill off foes? This thing is certainly beatable when you think about how many weaknesses it has depending on which route it chooses to counter pokemon. Here's a simple list of obstacles in the way of becoming 'broken' from both Routes. Note that this is by no means an exhaustive list.
Cons against taking Offensive Route: (Life orb + Magic Guard)
-Paralyze weak
-Burn weak (only if using physical attacks)
-Various EQs/Earth Power (due to weakened defenses)
*-Grass knot
Cons against taking Defensive Route: (Leftovers + Magic Guard)
-Much less offensive prowess
-Paralyze weak (again, but to a lessened extent)
-Very strong EQs (hard to take, regardless)
Magic Guard allows for a new spin in the customization of this pokemon in order to counter new threats and allows more depths to be explored. It is certainly possible that many players will simply use the offensive route, but there are multiple ways to use CAP10 by what I see: 2 excellent abilities, multiple items able to be employed successfully, stabs to work with, moveset of course.
In the end, I try to think most about the concept. Magic Guard heightens the amount of customization we can use effectively, and this is why it is a much better choice than Poison Heal.