Here is some rough theorycrafting on the matter that might be useful for people to visualise. I have read up on this thread and decided to have a crack at trying to account for all factors that abound from two different enchanting solutions. It is important to understand that i may have errors on a number of points, so i'd appreciate a critique of my assumptions. From here on in, E = Executioner, M = Mongoose. This is strictly for rogues, Warriors obviously will get a lesser benefit from M/M and a greater benefit from E/M, so the choice is simple.
For the Executioner Benefit %:
The DR% formula at level 70 is DR% = A/(A+10157)
The Delta DR% is therefore DRD% = (A-840)/(A+9317)
- These were taken from Wowwiki with the levels subbed in.
By determining ratio of these two, i was able to plot a relationship of Armor vs Executioner Benefit. My entire analysis was based on being unitless, so that you can attain factors to give an indication on how much better or worse you will be in a scenario with a particular enchant.
With 300 Paperdoll DPS:

** Just so we're clear - the plateau on both graphs is because ~4000 armor is taken off from common debuffs. Since negative armor doesnt have an increased effect, Executioner gets capped here.
For the Mongoose section, I started by using the Paperdoll DPS value on an armory sheet, since it accounts for your AP and weapon DPS. My Assumtions are:
* Hit cap is reached
* Specced into Lethality
* At all times CoR, 5xSunder and FF are up
* The rogue has about 33% crit when buffed in a raid
A factor of 6% was applied to determine the relative increase in damage from the critical increase from an additional mongoose proc. (When comparing M/M to E/M) also, the increase in haste and extra 120AP was accounted for. This particular figure i wouldnt mind a bit of help on. I went with a probability lineup and worked out how much crit 120 more agi gives for a rogue, then compared the damage assuming a lot of hits took place in a proc, then worked out as a % how much more that was without the proc.
To account for the haste increase on a mongoose proc, i reviewed a number of WWS logs and i estimated that ~ 55% of a rogue's damage is affected by the haste effect of mongoose. So in reality, that 2% from the haste only affects 55% of the damage. As a reminder, I'm using percentages and eventually factors, so that the data can be applied over a much broader set of scenarios.
Let's go with most bosses having ~7700 armor. Many sources will quote this. The mob has to be debuffed for this to be true but these are the values from the top graph.
PAPERDPS...175 200 225 250 275 300 325
FACTOR.....0.91 0.96 1.00 1.04 1.08 1.11 1.14
So in practice, you might create a hypothetical scenario where two identical rogues SS a mob. One had an E/M and one had an M/M arrangement.
At 325 Paperdoll DPS, if you had a M/M arrangement, and an SS hit for 500 - it would have hit for 570 had you had E/M.
At 175 Paperdoll DPS, if you had a M/M arrangement, and an SS hit for 500 - it would have hit for 455 had you had E/M.
As Paperdoll DPS increases, E/M increases
As Mob Armor Decreases, E/M increases
Again, please dont quote these as precise values, but this will hopefully clear up a bit of confusion and will help visualise the Two enchanting solutions.