This is always an interesting question
Which is best? This will depend on the work being done and the height working at.
If it is possible, rinsing off the glass will give the easiest way to achieve best rinsing results. However when working in difficult situations or at greater heights often rinsing on will become the easier and quicker way to work and still provide good results.
I tend to rinse off the glass at about 25ft and under. Over this height I will use one of two methods:
1. Partial rinsing on the glass - tilting the brush on to one side of the bristles, rinsing strips of the window across the glass supporting the weight of the pole on one edge of the brush bristles. Great for working at height where the glass needs a really good rinse, such as with heavy salt deposits.
2. Full rinse on the glass - Working in about 2ft wide strips down the window fully scrub the window, then starting at the top move the brush back and forth across the glass in the 2ft wide strip building a little head of water and then rapidly slide the brush down the glass in a slight zig-zag pattern allowing the head of water to follow the brush down the pane of glass. This way the last thing to touch each area of glass is the water not the bristles. Once the brush is at the bottom move it across at the bottom to the next strip of the glass slide it up to the top and start again.
However if I only have a few high levels panes of glass to do I will still rinse of the glass as I find it slightly quicker. Of course if you are working with any kind of physical impairment or injury it would be best to look at perfecting rinsing on the glass as this will greatly affect how much strain your body is under during the day's work.
It is worth bearing in mind that rinsing on the glass is very difficult to achieve with a fully flocked or a full natural bristle brush.