Difference between (split, align) and (gather, aligned)?

Question

Here is an MWE which shows that they seem to do the same thing. Is there any reason to prefer one over the other for typesetting groups of equations? I would prefer the align syntax because it allows intertext. In the other case, the alignment is limited to aligned environments within the larger gather environment. Any thoughts are welcome.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

%==================
% gather+aligned
\begin{gather}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{gather}


%==================
% align+split
\begin{align}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{align}

\end{document}

Answer 1

For the example alignment the following are equivalent; actually, the correct outer environment to use is equation, as the following code will show; using split or aligned for the inner environment in this case doesn't matter.

\documentclass{article}
\usepackage{amsmath,lipsum,geometry}
\geometry{margin=1cm}
\begin{document}
%\thispagestyle{empty}

\lipsum*[2]
%==================
% equation+aligned
\begin{equation}\tag{EA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{equation}
\lipsum*[2]
%==================
% equation+split
\begin{equation}\tag{ES}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{equation}
\lipsum*[2]
%==================
% gather+aligned
\begin{gather}\tag{GA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{gather}
\lipsum*[2]
%==================
% gather+split
\begin{gather}\tag{GS}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{gather}
\lipsum*[2]
%==================
% align+split
\begin{align}\tag{AS}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{align}
\lipsum*[2]
%==================
% align+aligned
\begin{align}\tag{AA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{align}
\lipsum*[2]

\end{document}

In the output it's possible to see that the correct spacing is obtained with equation. Use align or gather only when there is really more than one equation to number.

One should also note that "equation+split" results in a different vertical spacing than "equation+aligned" (which isn't really expected).

Answer 2

split can still work with narrow spaces while aligned cannot!

\documentclass{article}
\usepackage[a6paper,margin=1cm,showframe]{geometry}
\usepackage{pst-node,multido}
\usepackage{amsmath}
\begin{document}
\multido{\n=0.5+0.1}{4}{%
\endgraf\noindent\pnode(.5\linewidth,0){T}
%
\begin{equation*}\!
\begin{aligned}
\framebox[\n\linewidth]{aligned \n}
\end{aligned}\tag*{Def}
\end{equation*}
%
\begin{equation*}
\begin{split}
\framebox[\n\linewidth]{split \n}
\end{split}\tag*{Def}
\end{equation*}
%
\endgraf\noindent\pnode(.5\linewidth,0){B}
\psset{linecolor=blue}
\ncline{T}{B}
\pcline[nodesep=-\dimexpr\n\linewidth/2](B)(B)}
\end{document}