Veneretio has posted up his comparison of Mongoose vs Executioner. He does well enough to describe the pros and cons of each. In the end he had this to say about which one he feels wins out:
Neither of them. Myself I prefer Mongoose’s combination of Threat, Avoidance and Mitigation, but I’d also not discredit anyone who preferred Executioner for it’s superior Threat increase. Both are fine choices and unfortunately on this one, it’s all going to come down to preference.
While Veneretio may feel that neither of them is the clear winner, I feel that they are BOTH the clear winner! I would recommend getting each enchant on a different weapon and use those weapons in the proper circumstances.
I will drop Executioner on my [item]The Decapitator[/item] and keep Mongoose on my [item]King’s Defender[/item] and [item]The Sun Eater[/item].
When not currently tanking but needing to build aggro, swapping in Decapitator for Devastate is a very smart move when you find yourself with lower rage generation. If you are faced with the choice of getting only one enchant, I would still lean towards Mongoose. With the Threat buffs in 2.3, survival should trump threat in most fights when you are working on progression targets