This is exactly why Guts is a good thing. Not Flame Orb abuse, but the opponent activating it for you.
Let's imagine that CAP9 does get Guts, and is among the top 10 Pokemon in usage. It's near the beginning of the match, and you haven't had the time to scout the opponents team. It's your Blissey versus the opponents Latias.
In the standard metagame, Blissey wouldn't be too afraid of firing off a Toxic in the hope of crippling Latias for the rest of the match. But in a CAP metagame, one where there is a Guts user with Pursuit on the loose, she would be much less likely to do so. In short, CAP9 is stopping the secondary through mind games. It doesn't even have to be present as an active team member- the sheer risk of CAP9 switching in and getting a +1 attack boost is enough to deter people from using Toxic, Toxic Spikes, Will-o-wisp, and maybe more.
I think that Guts is a much better ability for this concept than the AMC. AMC reflects status (and other things) for a turn, and while that's cool, it doesn't do much to cripple many secondary users. Blissey has Natural cure; Heatran is immune to its Poison/Burn status; Breloom (if you are risky enough to switch in) doesn't get affected by its own Spore due to usually being Poisoned. AMC and Guts both stop the secondary in their own little ways, and I think both are needed to fully complete CAP9's purpose.
So, for those reasons, I voted Guts.