I would only go 3" if the engine is over 550 HP, or it is a race car. The 3" will be louder than the 2-1/2" if using the same type mufflers. The mandrel bent exhaust does flow way better than the standard crimped bends. The rest comes down to cost and looks. Stainless should last longer and can be polished up for a show car, but is more expensive than the aluminized exhaust. Custom bent would be best if the person does a good job, but this again usually is more expensive than buying a kit off the shelf.