The debate is has continued for eons and probably will for eons more. On one side, underwires create definition but at what cost? If incorrectly fitted they can be extremely uncomfortable (and a downright pain!) It is even debated as to whether they can contribute to breast cancer?
Fans of non-underwires make the decision fora more natural appearance to the breast with a greater comfort level for the wearer.
The choice is yours. Maybe non-underwire for casual times and underwear for more formal occasions?
Do your research to see what options are best for your particular body type.