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
The 3rd option is by far the best through the first 3 difficulties and evens out in Inferno due to the reduction.
They each have their own requirement for being practical:
- Attack Faster!
- Spend Spend Spend!
- Hit 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.
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 moderating 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... +50% resists rune please!