The payoff/reward is much bigger in terms of happiness

[angliholic]

So invest your time and energy in people. The payoff is much bigger in terms of happiness.
So spend your time and energy in/on people. The reward is much larger as regards/with reference to/in respect of happiness.


I wonder if I could reword the first sample as the second. Correct me if I am wrong.

[Niall]

to spend (something) on means that on is the appropriate preposition here.

I would tend to use the word greater rather than larger. Simply because larger is used more often to describe physical size, though it isn't wrong to use larger here.

in respect of sounds best here from your options, but still doesn't sound natural. Perhaps you could use the phrase in terms of. :)

So spend your time and energy on people. The reward is much greater in terms of happiness.

[angliholic]

Thanks, Niall, for your most desirable version.
I like it so much that I can't help but write it one more time--So spend your time and energy on people. The reward is much greater in terms of happiness.
It occurred to me just now that it seems all right to write "as far as happiness is concerned" instead of "in terms of happiness." What do you think?

[Niall]

Yes, I like that. :)