If you have two separate power sources you must choose a transformer vector to make the two sources match. For example you may have a primary source at 35kV and have a backup from a 12kV distribution system. It is likely that these two are not in phase with each other so you must choose transformers to make them match if there is any possibility of paralleling them or making a closed transition transfer.
If that is not an issue then you should buy what is standard from the manufacturer. If you buy from a US manufacturer you will most likely get a unit with the high side leading the low side by 30 degrees (ANSI standard) while IEC manufacturers more commonly have the Delta side lead the Wye side by 30 degrees (Dy1 or Yd11).
Suggestion: The neutral is often being dealt with the vector group. Therefore, it is necessary to find out which vector group to select to match the one line diagram power distribution engineering including the system grounding.
