1. No pain, no gain. 2. No pains, no gains.

Which do you think is correct?
Haven't heard 2 too much.
I think pain and gain are uncountable.
Students: We have free audio pronunciation exercises.
 Marius Hancu's reply was promoted to an answer.
To feel the growing pains is something most companies have to go through.
I am at great pains to obsequiously solicit the favors of the guests.

When it comes to joining this forum, the gains in terms of how much progress you will make are substantial.
Not to mention other gains: sense of achievement that you draw from helping the others, etc.

No pain, no gain, however.