Really comes down to size of the vessel. Horizontal vessels are better suited to holding larger volumes of liquid than vertical vessels. If your gas/liquid ratio is very small, a horizontal vessel can be run with a fairly high liquid level and stay a reasonable size (you can even put a small vertical dome on it for the mistex), while with a vertical vessel, either the diameter will get excessively large or the liquid height will get excessively tall to handle the liquid holdup. For low liquid levels or vessels that run normally dry, you can size the vertical vessel more economically than a horizontal. Try playing with a few different cases and see how the sizes stack up. There are some cases where either will work equally well. Sometimes all this is overridden by other constraints such as client preference, available space, etc. The key is to be flexible, investigate your options and client requirements/preferences, based on my experience there are almost no hard and fast rules, equipment design should be "fit for purpose".

Cheers,