More agony resistance from infusions

Assuming Guild Wars 2 is similar to other games, its still early so the exact combat formulas aren't well understood, but usually its a trade off where vitality (HP) dominates early on, but once you have enough HP, toughness/defense/armor becomes more important.

The metric you want to use to maximize survivability is effective hp which takes into account both vitality and toughness. The same goes with power/precision for calculating max DPS.

There are some trade-offs. Overall, I consider Vitality to be a little better, but it will depend on your build, as well as the enemy you're fighting.

Toughness

• Toughness reduces the amount of damage you take from a given attack. Builds that focus on toughness will have a smaller health pool, but receive less damage to make up for it.
• The main disadvantage of stacking toughness is that some attacks ignore it. This includes damage over time from conditions.
• The main advantage of toughness is that it keeps your health pool smaller, allowing you to heal more efficiently.

Vitality

• Vitality increases your health pool. Builds that focus on vitality will take more damage than toughness builds, but are able to absorb more before dying.
• The main advantage of vitality-heavy builds is that the extra health cannot be circumvented, like toughness can.
• The main disadvantage of vitality is that a huge health pool is more difficult to heal up.

Conclusion

It's probably wise to have some mix of these two stats. In general, I would favor Vitality. Of course, the more you have of both, the more tanky you will be.

If you have ways to mitigate damage-over-time conditions, Toughness might be advantageous, since you can avoid much of the downside. Likewise, if you have very poor healing ability, you might want to stack Toughness so you don't have a massive health pool you're unable to refill.

If you have ample heals, especially some form of continuous healing, Vitality will probably be more advantageous, as you can keep that huge health pool refilled.