Which is the best when advertising in facebook, pay per click or impressions?

[MH]

I'm trying to advertise my group on facebook, it's a group for selling things.
I have made an advertising on facebook with pay per click method, I have 0.15 max bid and 4$ max daily budget....but still not getting too much memebrs in my group.

Will pay per impressions be better than pay per clicks? what is the difference?
If choose pay per impressions, will I have to pay when people click on my ad? or just see it?

Regards.

[Bella]

I also advertise on facebook and do the pay per click. I like pay per click better than impressions because while fewer people might see it, clicking it will get them to the site rather than them just seeing my ad.