Beamer - split frame into two columns and adjust itemize environment

See, if the following solution gives what you looking for:

\documentclass{beamer}

\begin{document}
\begin{frame}{Comparison}
    \begin{columns}[t]
\begin{column}{0.48\linewidth}
        Pros:
        \begin{itemize}
            \item<1-> 1
            \item<2-> 2
            \item<3-> 3
        \end{itemize}
    \end{column}
    \begin{column}{0.48\linewidth}
        Cons:
        \begin{itemize}
            \item<1-> 1
            \item<2-> 2
            \item<3-> 3
        \end{itemize}
\end{column}
    \end{columns}    
\end{frame}
\end{document}

If you want to have fine control, specify the frames explicitly instead of using \pause and <+-> (see version 3 below).

Version 1:

\documentclass{beamer}
\begin{document}
\begin{frame}{Comparison}
\pause
    \begin{minipage}[t]{0.48\linewidth}
\onslide<2->%
        Pros:
\pause
        \begin{itemize}[<+->]
            \item 1
            \item 2
            \item 3
        \end{itemize}
    \end{minipage}
    \hfill
    \begin{minipage}[t]{0.48\linewidth}%
\onslide<2->%
        Cons:
        \begin{itemize}[<+->]
            \item 1
            \item 2
            \item 3
        \end{itemize}
    \end{minipage}
\end{frame}
\end{document}

Version 2:

\documentclass{beamer}
\begin{document}
\begin{frame}{Comparison}
\pause
    \begin{minipage}[t]{0.48\linewidth}
        Pros:
\pause
        \begin{itemize}[<+->]
            \item 1
            \item 2
            \item 3
        \end{itemize}
    \end{minipage}
    \hfill
    \begin{minipage}[t]{0.48\linewidth}%
\onslide<6->%
        Cons:
\pause
        \begin{itemize}[<+->]
            \item 1
            \item 2
            \item 3
        \end{itemize}
    \end{minipage}
\end{frame}
\end{document}

Version 3:

\documentclass{beamer}
\begin{document}
\begin{frame}{Comparison}
\pause
    \begin{minipage}[t]{0.48\linewidth}
        Pros:
        \begin{itemize}
            \item<3-> 1
            \item<4-> 2
            \item<5-> 3
        \end{itemize}
    \end{minipage}
    \hfill
    \begin{minipage}[t]{0.48\linewidth}%
        Cons:
        \begin{itemize}
            \item<3-> 1
            \item<4-> 2
            \item<5-> 3
        \end{itemize}
    \end{minipage}
\end{frame}
\end{document}