It's good practice to always use the [] notation. One reason is that attribute notation (df.column_name) does not work for numbered indices:
In [1]: df = DataFrame([[1, 2, 3], [4, 5, 6]])
In [2]: df[1]
Out[2]:
0 2
1 5
Name: 1
In [3]: df.1
File "", line 1
df.1
^
SyntaxError: invalid syntax