You will be struggling to find any information on the WSV13. The SV indicates it is a "special version"/"special variation" (no-one's sure exactly what it stands for) of a standard model. Singer made a lot of these SV versions of different models but what they were for is anyone's guess. The W indicates it was made at the Bridgeport USA plant but that burnt down with the subsequent loss of most of their records.
Having both would be nice (if the price is right) as a cylinder arm can be useful, but if I could only have one it would be the 155, it is quite highly regarded, is a reasonably heavy duty machine and feet, bobbins etc are readily available.
You also don't mention what you want to make (If you say holsters then neither would be suitable!!).