The \hfill
solution works in this particular case only because identical terms are used as the first term for both the denominator and numerator. For more general cases, this may not quite provide the desired alignment. For example, if the numerator contained w_k
and the denominator c_k
, using just \hfill
would provide the alignment as illustrated by the last example below.
This solution provides an \AlignMe
macro that has one optional parameter and two mandatory parameters:
\AlignMe[<alignment>]{<content to determine space>}{<content to be typeset>}
where <alignment>
can be the usual single letters of l
eft, c
enter, and r
ight.
Below are a few examples illustrating this and comparing to the \hfill
solution where the alignment of the first term is adjusted and the alignment of the minus sign remains intact:

Notes:
- You should use
\text{if}
instead of if
as Hendrik Vogt commented.
- My personal preference would be equation (1). Even though that results in the denominator not being centered with respect to the vinculum being
Code:
\documentclass{article}
\usepackage{xcolor}
\usepackage{amsmath}
\usepackage{calc}
% Add color here to simplify comparrison of the alignment
\newcommand{\CK}{\textcolor{red}{c_k}}
\newcommand{\CkBar}{\textcolor{blue}{\overline{c_k}}}
\newcommand*{\AlignMe}[3][c]{%
\makebox[\widthof{$#2$}][#1]{$#3$}}%
\begin{document}\noindent
Using \verb|\AlignMe[r]| for first and \verb|\AlignMe[l]| for second term:
\begin{equation}
p_k(s)=
\begin{cases}
\dfrac{w_k -q_k(s)}{\AlignMe[r]{w_k}{\CK} - \AlignMe[l]{q_k(s)}{\CkBar}}
& \text{if } q_k(s) > \overline{c_k} \\
1 & \text{if } q_k(s) \leq \overline{c_k}
\end{cases}
\end{equation}
%
Using \verb|\AlignMe[r]| and \verb|\hfill|:
\begin{equation}
p_k(s)=
\begin{cases}
\dfrac{w_k -q_k(s)}{\AlignMe[r]{w_k}{\CK} - \CkBar\hfill}
& \text{if } q_k(s) > \overline{c_k} \\
1 & \text{if } q_k(s) \leq \overline{c_k}
\end{cases}
\end{equation}
%
Using \verb|\AlignMe[r]| for first and \verb|\AlignMe[c]| for second term:
\begin{equation}
p_k(s)=
\begin{cases}
\dfrac{w_k -q_k(s)}{\AlignMe[r]{w_k}{\CK} - \AlignMe[c]{q_k(s)}{\CkBar}}
& \text{if } q_k(s) > \overline{c_k} \\
1 & \text{if } q_k(s) \leq \overline{c_k}
\end{cases}
\end{equation}
%
For comparrison, using \verb|\hfill|:
\begin{equation}
p_k(s)=
\begin{cases}
\dfrac{w_k -q_k(s)}{\CK - \CkBar\hfill}
& \text{if } q_k(s) > \overline{c_k} \\
1 & \text{if } q_k(s) \leq \overline{c_k}
\end{cases}
\end{equation}
\end{document}