Project Euler 6: Difference between sum of squares and square of sum
Looks good to me.

Couple of things to consider though:


You could split out two functions, squareOfSum and sumOfSquares. I think that would make it a little easier to read.
You could make the functions more general by using Seq.sum/Sum.sumBy instead of List.sum/List.sumBy.
While you could use fold, as joranvar mentioned, I think this would make the program much less clear and I would not recommend it (except as an exercise for learning about folds).
There is another way to attack the problem, by using the two identities


\begin{align}
1 + 2 + 3 + \cdots + n &amp;= \frac{n(n + 1)}{2}, \quad \text{and} \\
1^2 + 2^2 + 3^2 + \cdots + n^2 &amp;= \frac{n(n+1)(2n+1)}{6}.
\end{align}