legendary armor set

Depends on what you're going to be doing.

If you intend to do PvP i'd suggest you collect sets of each so you can swap and change depending on what class / who you're up against.

For PvE you'll do well to have some craft gear, about 1 or 2 sets again depending on what you're facing (especially when it comes to CCBD)

Personally I would go for the one with the Constitution effect, but that's just based on my character and his build xD. It really all depends on you and your character, just like what Cosmo said.

For PvE,

Higher LP ☞ LP ABS is better

Lower LP ☞ CON is better

Basically, for classes with a high LP, LP ABS is way more efficient than CON. If you are not a tank, CON is better for playing dungeons.

One should keep in mind, however, that LP ABS cannot prevent you from getting killed by one crit hit if the dmg is greater than the LP you have at the moment of receiving the hit, as you presumably already know. For example,

(1a) 10,000 LP + 56 CON = 14,480-15,600 LP

(1b) 10,000 LP + 42% LP ABS = 14,200 LP

If hit by a skill of 11,000 dmg, (1a) will survive, with having around 4k LP, whereas (1b) will die.

(2a) 20,000 LP + 56 CON = about 25,000 LP

(2b) 20,000 LP + 42% LP ABS = 28,400 LP

If hit by a skill of 19,000 dmg, (2b) will survive with a more LP left. Plus, it can be full-healed by a dende or recovered using LP potions, much more easily and efficiently. Notice that (2b) will die if the dmg is greater than 20,000. And that's why it is better for some to gear with CON while fighting against some strong dungeon bosses.

For PvP, it depends, as others have already mentioned.

To clarify what I wrote right above,

Let's say you have a 20k LP with buffs and are geared with 42% LP ABS in totall on your armours.

Suppose you first get hit by a boss with a dmg of 10k. The final dmg you will get from the hit should be 5,800 (10k x (1-0.42)).

Your remaining LP is now 14,200. Before you get healed by the dende in your party, suppose you get hit again with the same dmg from the boss. Then, your remaining LP will be 8,400.

If you are attacked one more time by the same attack from the boss before getting healed, you will die because the original/initial dmg (i.e. of 10k w/o ABS) is greater than your remaining LP (which is 8,400 in the case being discussed now) although the actual dmg you would get (i.e. 5,800) is smaller than your remaining LP (i.e. 8,400).

Most people know this, so this is only for those who don't.