Hi Sue, your choice is usually ruled by a number of factors. Personally I do not varnish my acrylics as I do not feel that they need it and, after many years, they are still in the good shape they started out in.
If you do varnish, you need to decide what effect you want to achieve. For larger paintings, matt is certainly best (in most cases) as the high shine that can come off a gloss or even satin surface in the light can make the image hard to see. Matt or satin are usually the best choices if you are looking simply to seal and protect, and matt leaves a flatter finish with less light reflecting properties. However, if you want to varnish for effect, for example to give water a shine, then your glass would be a good bet. I know some artists who like to create a textured effect with their varnish, so they varnish a sea-scape, for example, in matt or satin, and then tip the waves in gloss. But most who varnish their work are looking simply for a flatter protective finish that is easy on the eye. Which is where matt becomes your best friend.
Whatever you decide to do, make sure that you photograph your work before the varnish goes on, as the varnish can distort what the camera picks up.
Hi Meg, thank you very much for your reply, that is really helpful :)