My 2 bob.... Go the single. I've gone from a twin to a single and glad I did. 2.5" to the muffler box and 3" from there out the back. My 253 has a very mild cam and the note was just smoother out the single. The 3 inch out the back lets you scare the birds when you need, but ride the throttle right and it drops down to a nice burble. Pay the dosh on a good sports box and it will make the difference and not sound like some try-hard with a straight-through.
The only con's was missing the look of the twin chrome droopys and the dual puffs of steam out the back on cold mornings.