Depends on the plane and your budget. For my Focus for next year I am using a two stroke. The airframe has a enough drag built in, tends to be a little heavy and likes the power. For my Prophecy I will keep my YS. A clean plane like this is too fast with a two stroke.
Fuel consumption is similiar since two cycle are loafing most of the time. Many are switching for simplicity and cost with the added benefit of less weight and more power.
Watch what people are flying at your local contest learn from others experience.