One more thing to consider -- the direction of the exhaust tips.
Downward, you get even distribution of the noise. Cars ahead and beside you will hear you better than just the cars behind.
Side, obviously, the side where the tip is pointing gets most of the sound. If you have a wall on one side and a car on the other, the noise gets distributed much better. The same with having cars on both sides.
Rear, the cars beside you and ahead you will not hear the real volume of your exhaust. You might think you are NOT loud enough, but you could probably be heard a mile away.
Me, I like down or side for safety reasons and also so that I can enjoy that exhaust note better. My friend has downward straight pipes, and it hurts my ear regardless of where am I around his bike, even with a full face helmet. Mine is a side exhaust pointing to the right, so I stay on the left side of the road as much as possible for better auditory presence.
You should check Big City Monster Baffles (www.bigcitythunder.com). I have one and they are not kidding about giving you a straight pipe loudness without losing the backpressure for low end torque. How true? I can wheelie my bike anytime and I have Bub Jug Huggers with Monster Baffles. Fuel programmer like the Power Commander also affects loudness as well as any intake help like K&N filters or Big Air Kits. Check mine in youtube...
http://www.youtube.com/watch?v=k7iQMKCpMMY
http://www.youtube.com/watch?v=tZrpQ77iUE8