What are the pros and cons of each life gaining stat?
It depends on your style specifically as others have stated. You also left out another life gaining option.
- Life on Hit
- Life on Spirit Spent
- % Damage to Life
- Life Regeneration
The 3rd option is by far the best through the first 3 difficulties and evens out in Inferno due to the reduction but then scales back up once you have end game gear (dps > 100K).
They each have their own requirement for being practical:
- Attack Faster!
- Spend Spend Spend!
- Hit Harder!
- Tank Harder!
That said... they are all useful. Option 1 will probably give more survival if you are "Tanking" a group, but if your current items lead you to a kite & hit strategy, it is possible that Options 2 or 3 would be better for you. Option 4 also works well with a kiting strategy and is better with higher damage reduction to life ratio.
Obviously, there is nothing wrong with having a moderate amount of each of these, but you also need to increase their effects by increasing your EHP. Increased EHP and lower HP results in heals being much more effective.
Also... If you like the Life on Spirit Spent option, another strong modifier to consider is Spirit Regen / Second.
A combination of Sweeping Winds (+3 spirit per second at full stacks) and the Passive that will increase your Mantra's duration and give you +2 spirit per second and 1-2 items with 1-2 spirit per second can allow you to get close to 10 spirit regen per second, without attacking.
This allows for increased value from Life on Spirit Spent.
Again, life gain without EHP is virtually useless... Stack up you ALL RESISTS > 1,000 if possible and watch how much easier inferno gets... heaven forbid you do that before your barbarian friend WARCRIES... +20% resists rune please!
They're completely different and are used differently depending on your desired build. For example, Life on spirit spent focuses on burning your spirit for quick heals, and life on hit requires faster attack speed to emphasize drawn-out survivability. If you have an idea for a specific build, that would be helpful.
As is, both seem to be required for Inferno, and both required a large amount of Effective Health, with a lower amount of Actual Health.
Both can be very useful, but it depends on your build and play-style.
Life on Hit supports a sustained, fast-attacking playing style. This includes using fast weapons such as daggers/fists, dual-wielding, or using items/skills that increase attack speed. Also, since you need to be actually hitting monsters to regain life, you need a decent amount of survivability to be able to tank the monster's damage (even with high LoH).
Life on Spirit Spent goes well with kiting/burst-damage builds that have high passive spirit regeneration, and use of spirit spenders to manage monsters (e.g. crowd control skills such as Blinding Flash, or the knockback of Lashing Tail Kick).