My first love is sketching live on location and there is no better result than being there, on location, capturing the scene. There is something very true about the result. It may not be a photographic representation but the spirit of the scene is evident. These are the best results in my opinion.
But whilst I love sketching live on location, I can sketch from existing photography if the quality is good enough. The results from these commissions are often visually as good as the live sketches and the finished results are as good as live sketches.