A simply phrased question that has a very complex answer.
To get into which is better, you first need to ask yourself the following. Do you want to avoid more hits or mitigate more total damage? In general I have valued mitigation more so than avoidance. So even if 2 different stats are statistically similar, or cause your character to take the same amount of damage over time, I still prefer the mitigation.
This is because I believe that mitigation allows any particular set of swing to each be more closer to the average swing (assuming a miss is a hit for 0). This basically mean, you will take damage at a more consistent rate.
Taking damage at a more consistent rate is easier to be healed through. Also note, the faster the mob hits, the more close avoidance will be to mitigation in terms of consistency for a given length of time.
HP also comes into play, but most of the benefits should be pretty obvious, and I don't feel the need to address the more complex issues in great detail at this time.
To define an avoid: Parry, Riposte, Block, or any skill that causes a swing at you do 0 damage is an avoidance skill. Luckily for you, these are parseable. Any time you parry, riposte, block, etc, your log file reports it as so.
You will need to do one of the following calculations yourself to most accuratly rank the worth of the skill mods to your character:
Way #1 - The Controlled Way
I've never parsed it myself, but this is what I would do if I were interested:
1)Get a good parser, and find a mob (and always the same mob) and let it swing at you for 10-15 minutes.
2)Calculate the % parry, riposted, blocked, dodge etc. Some of the parsers do this for you automatically, for example gamparse does.
3)Add in some parry, riposte, block, dodge mod etc and only one of different types, and do the same test again. Note the % increase in avoidance.
4)Do the same test again at X and Y ac values. Note the difference in average hit against you.
*NOTE* Also record the average dps you took over the period and the total damage taken. For greater accuracy, repeat these tests. The more times repeated, the more accurate the results.
Way #2 - The Lazy Way
1)I parse my huge log file, and figure my average % avoid based on each skill.
2)I then take my character skill in each ability and divide the mod on an item by this number. This gives me a % increase.
3)I multiply the average % avoid by the % increase (step1 x step2).
Example w/ made up numbers:
1)I riposte 10% of attacks
2)I have 225 riposte and the item in question has a 5 riposte mod. So it would gives me a 2.22% increase in dodge skill.
3) 10%x2.22%=10.22%
So I gain a 0.22% increase in avoidance in this example. Or, .044% increase/pt of riposte mod.
I then rank this against the other stats of the item.
I value ac (fomelo) at the rate of a minimum of .0333% mitigation per point of ac.
So in terms of equal benefit for this example: 5 riposte = 0.22% increase in avoidance = 6.6 ac = 0.22 increase in mitigation. But I already stated, I value the mitigation higher, and not only that the AC is only a minimum of .22 increase in mitigation to me.
I'll let you figure out the worth of HP (most people value it at 1 ac = 3 - 5 hp), but this varies depending on spell damage taken, etc. You should also take into consideration the benefit you gain from the riposte itself doing extra damage, having a chance to proc, and generating more agro on the mob you are tanking.
If you feel you are unable to do these calculations, send me some of your logs and I would be glad to assist you with my "lazy way". I am also more than happy to answer any other questions you have. If you want to know what I value each mod at for my character, feel free to send me a PM here, a tell in game, or come on our vent.
