I prefer Sunday.
Reason: The intention of Saturday evening mass was for people who for legitimate reasons could not make mass on Sunday. Before this was allowed, it wasn’t uncommon for some parishes to have one or two mass between the hours of 12AM (midnight) to 6AM on Sunday in order to accommodate people who could not make a normally scheduled Sunday mass due their work schedule.
Some of these masses, esp the 2 to 4 AM time slots were often called the “newspaperman’s Mass,” because it was for people who worked for the local newspaper.
The Saturday post Vespers Mass was created to prevent Priests (and laity) from attending Mass at those crazy hours.
However, unfortunately today, what has happened is the Saturday evening mass SEEMS to be mostly filled with people who simply don’t want go in the morning.
Also, unfortunately, I think too many priests schedule the Saturday evening mass without determining what time slot is best for the people who actually work on Sundays.
Truth be told, I would imaging that in many parishes, having a Sunday evening mass would be better than a Saturday evening mass.
God Bless.