Which is better printf or echo?

Preferable and most widely used is not the same thing. While printf is better for many reasons, most people still use echo because the syntax is simpler.

The main reasons why you should prefer printf are:

1. echo is not standardized, it will behave differently on different systems.

2. It is hard to predict what you're actually running when you echo foo. To illustrate, on my Debian system:

   $ type -a echo
   echo is a shell builtin
   echo is /bin/echo
   

   As you can see, there are two different echo commands, one is a shell (bash in this case) builtin and another is a separate binary. Note that bash also has a printf builtin but its behavior is more standardized so it is less of an issue (thanks to @ RaduRădeanu for pointing it out).

3. Since some (but not all) implementations of echo support command line switches, it is hard to print a string that starts with a -. While many programs support -- to signify the end of switches and the beginning of arguments (for example, grep -- -a file will find lines in file that contain -a), echo does not. So, how do you have echo print -n?

   $ echo -n           ## no output
   $ echo '-n'         ## no output
   $ echo "-n"         ## no output
   $ echo \-n          ## no output
   $ echo -e '\055n'   ## using the ASCII code works but only on implementations
   -n                  ## that support -e
   

   printf can do this easily:

   $ printf -- '-n\n'
   -n
   $ printf '%s\n' -n
   -n
   $ printf '\055n\n' 
   -n
   

For more information than you ever wanted to know on why printf is better than echo, see this answer to a similar question on http://unix.stackexchange.com:

https://unix.stackexchange.com/a/65819/22222

To ask which is preferable is itself incomplete.

If all one wishes to do is emit one or more lines of text terminated by newlines, then echo suffices. If anything more clever is intended, specifically including a "partial line" that does not have the newline, then printf is best for that purpose.