What's the difference of using #!/usr/bin/env or #!/bin/env in shebang?

#!<interpreter> <arguments> tries to run <interpreter> <arguments> to read and run the rest of the file.

So #!/usr/bin/env means that there must be a program called /usr/bin/env;
#!/bin/env means that there must be a program called /bin/env.

Some systems have one and not the other.

In my experience, most have /usr/bin/env, so #!/usr/bin/env is more common.

Unix systems will try to run <interpreter> using execve, which is why it must be a full path, and #!env without a path will not work.

Mikel explanation is great, it misses just a small fact (which is rather important), it's only one argument being passed including all spaces:

#!<Interpreter> <argument>

Results in calling:

$ <Interpreter> '<argument>' path_to_calling_script

So for Example:

$ cat /tmp/test
#!/usr/bin/env python
print "hi"

$ /tmp/test

is the same as calling:

$ /usr/bin/env "python" /tmp/test

The quotes try to show that if you add any flag or other values will be part of the argument being called.

 #!/bin/bash -c /bin/env python

Will be interpreted as:

 $ /bin/bash "-c /bin/env python"

Which won't work.

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