2 into 1 usually gives more power simply because the pressures are balanced. A true dual can be better if it can provide the same flow and pressure on both cylinders. Tuning and the mufflers have an affect. Sound is usually of great importance so true duals might sound better. It is difficult to bend the front header and rear header in a way that the air flow is equal. Generally it is not worth spending money on true duals for a bike in my opinion. Some will disagree. Maybe a tuner like Craig Bennett has found dual headers that allow a better tune. I sell a number of brands of pipes and the manufactures have told me that their 2 into 1 pipes are almost alway better. Again, someone like Craig might know of a set up that is better. If so, you would want to work with him or someone that can get the right tune for you.