From an experienced property manager that handles both single family and multi family, the answer to this question will depend on what your investment goals are. If you are looking for appreciation in value, typically single family homes have been good performers. If you goal is income today, then you will want to consider a multi family. I would be happy to help you explore some options as I have for many other clients that have recommended my on Trulia.com profile.
The market is very strong and rental pricing continues to increase which is why you see so much competition between investors for both single family & multi family homes. You definitely need to be savvy in order to make the right decisions and compete in this type of market. If you are not, you can dig your self a hole that could be difficult to get out of.
As Jenni said... it depends. The question offers no insight into you skills, goals, background and other information needed to give a valid answer. Making a living as an investor/landlord is a difficult profession requiring a lot of education and forethought. It's no wonder so many "investors" lost their shirts in 2007-2010... they thought being a professional investor was easy.