Which glows more tonic water or highlighter?

Neither of these two examples glows of their own right. Both materials reflect light waves that impinge on them. [hint. consider looking at them in a dark room.]

These two materials fluoresce due to their component matter fluorescing under ultra violet light. Thus they appear to be reflect more light than is incident.

You can easily demonstrate this by putting an UV absorber between the light source and the object.

And as to the question - the highlighter is designed to draw attention to the mark - an important property. So we would expect the fluorescence of tonic water to be rather unimportant, whereas it is a design property for the highlighter.