You can find a detailed article on this subject here: https://investfourmore.com/2013/04/19/is-it-better-to-fix-and-flip-houses-or-buy-and-hold-rentals/
I have flipped over 150 houses and bought 20 rentals. I love flipping and owning rentals, but which one is better? I think it is hard to compare the two strategies because they are both so different. Flipping houses is more of a job, and holding rentals is more of an investment. My strategy is to make money with house flips so that I can invest more money into rental properties.
Overall, I think the best way to invest is in rentals, but the more money you make, the more rentals you can buy. It helps to have a high paying job which flipping houses can provide. I have switched to buying more rentals or buying more houses based on the market as well. Some markets are better for flipping, while others are better for renting.
The market in Colorado took off the last couple of years making it tough to make money with residential rentals. However, I found that commercial rentals have better returns in my area and I have been investing in them lately. I am still flipping houses and buying rentals.
I think investors need to be careful with what properties they buy as rentals because you will have the property longer. Here is a great article on what to repair on a flip or rental. https://investfourmore.com/2013/10/02/how-to-repair-investment-property/
Here are some of my books that go into more detail on both.
Build a Rental Property Empire:
Fix and Flip Your Way to Financial Freedom