To make things a little more simple:
Guardian:
* Build Diversity: High - just like any other profession in the game, they have the most potential for build crafting because of how you can mix and match your healing, utilities, and elite.
* HP: Lowest - they have the lowest HP in the heavy armor class. You have to rely on blocks and multiple sources of healing to keep you alive. You have to be mindful on how you also pick your stats when gearing.
Revenant:
* Build Diversity: Limited - Revenant legends are "presets" that is based on the principle of the dynamic combat of GW2. When you pick you your legend, you're picking a "theme/playstyle" that represents either power, condition, healing, defense, profession specific mechanic, boons, support. Each set will have their own healing, stun break, crowd control, and elite skill.
* HP: Medium - they have a moderate amount of base HP, which gives you more leeway to spec more on aggressive stats. Revenant's sustain comes from making sure you have the enough energy to use your healing skill or support skills.