A powered approach will generally be a flatter approach and appears to be the normal approach a most airfields. They appear to be flown a bit faster and a the circuit a little wider as a result. At many busy GA airfields this will allow more traffic to be circuit at the same time. So far all of these are upsides for the flying club and airfield operator. One of downside happens when it all goes quiet up the front. Where will you go? The other is when conforming to a traffic pattern, you'll often conform to the speed. In my opinion, most G/A aircraft are flown far to fast on approach. Long runways mask the real problem and that is, you may not have learnt and practiced the correct technique for performance landings. So when you HAVE to do one...
A glide approach is a nifty procedure for keeping the circuit small and tight and the noise to a minimum. When flown correctly, you are insured against engine failure (by definition) and the result will generally be well aimed touchdown and a short ground roll. The downside will often be seen when someone pulls one of these out of the bag, unannounced, in the middle of a crowded circuit. Potentially they'll be descending over the top of people on finals and be totally unaware of their presence. Cooling could be another problem as could carb. ice under the right (wrong?) conditions.
Give me the choice, and I go for something between the two. But no matter which approach you do, getting the picture and numbers right is what its all about.
PM