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

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).

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}