Chunky vs. Smooth Strings Consider a string of length N, such as Peanut Butter with N = 13. Notice that there are N-1 pairs of neighboring characters in the string. For Peanut Butter, the first of the 12 pairs is Pe, the second is ea, the last is er.
When the pairs are mostly different characters, the string has a chunky quality, e.g. chUnky.
When these pairs are mostly the same character, the string has a smooth quality, e.g. sssmmsss.
Define the chunkiness of a string to be the ratio of the number of pairs with two different characters to the total number of pairs (N-1).
Define the smoothness of a string to be the ratio of the number of pairs with two identical characters to the total number of pairs (N-1).
For example, Peanut Butter only has one pair with identical characters (tt), so its smoothness is 1/12 or 0.0833 and its chunkiness is 11/12 or 0.9167.
Empty strings and strings with only one character are defined to be 100% smooth and 0% chunky.
Challenge
Write a program that takes in a string of arbitrary length and outputs either its chunkiness or smoothness ratio as a floating point value.

Take input via stdin or the command line, or you may write a function that takes a string.
You can assume the input string only contains printable ASCII characters (and hence is single-line).
Print the float to stdout to 4 or more decimal places, or you can choose to return it if you wrote a function. Decimal places that convey no information are not required, e.g. 0 is fine instead of 0.0000.
Choose chunkiness or smoothness as you prefer. Just be sure to say  which one your program outputs.

The shortest code in bytes wins.
Examples
Peanut Butter → Chunkiness: 0.91666666666, Smoothness: 0.08333333333
chUnky → Chunkiness: 1.0, Smoothness: 0.0
sssmmsss → Chunkiness: 0.28571428571, Smoothness: 0.71428571428
999 → Chunkiness: 0.0, Smoothness: 1.0
AA → Chunkiness: 0.0, Smoothness: 1.0
Aa → Chunkiness: 1.0, Smoothness: 0.0
! → Chunkiness: 0.0, Smoothness: 1.0
[empty string] → Chunkiness: 0.0, Smoothness: 1.0
Bonus question: Which do you prefer, chunky or smooth strings?
CJam, 19 bytes

q_1&gt;_@.=:!1b\,d/4mO


100% chunky source code that calculates chunkiness.

Try this chunky goodness online.

How it works

q_                  e# Read from STDIN and push a copy.
  1&gt;_               e# Discard the first character and push a copy.
     @              e# Rotate the original on top.
      .=            e# Vectorized comparison (1 if equal, 0 if not).
        :!          e# Mapped logical NOT (0 if equal, 1 if not).
          1b        e# Base 1 conversion (sum).
            \,      e# Swap with the shortened string and push its length.
              d/    e# Cast to double and divide.
                4mO e# Round to four decimal places.


Obviously, NaN rounded to 4 decimal places is 0.