Now I will at least toss on my knowledge of the 2 with experiences to give you an idea of how I've used both in the past, you can share your own experiences if you'd like.
Cost Per Action - I've found that these may convert very high, but when it comes to profitability you need to reach a very large market.
I've really been able to do well with this with the backing of social networks like Facebook and YouTube. Sometimes even converting over 25% of leads to fill out their email address. But again, I was only making around $1.00 - $2.00 per person who took action on the offer.
Cost Per Lead - Oddly with the experience I've had these have also converted very high, you don't need a large market if you can just focus on extremely targeted leads.
My experience has been best with search engine and paid traffic. With very little traffic I have still converted above 10% on many occasions. My best experiences were when I helped a company increase their clients. However, I was not paid unless they actually turned that lead into a sale. But the profits from it were above $300 - $500.
So from my personal experiences CPL has done extremely well with much less work.
What are your experiences?