The simple answer: If it’s an easy action to take, like “sign up for our newsletter” a lead form usually works well. If it’s a complicated product that you’re selling, a landing page usually works better.
The better answer: Run an A/B test AND (this is the important part) track everything to see what ultimately nets you more money.
For example, we had a client who was A/B testing lead forms and landing pages. The cost per lead was lower for the lead form. HOWEVER, those leads were less likely to lead to appointments and actual sales. It turns out the landing page leads ultimately led to 3x more revenue. Without testing AND meticulous tracking all the way through the sales funnel (which required a fair amount of effort on the client’s side), we would not have known.