The Bell Star is designed more for a tucked/racing position as it is a race helmet and the RS-1 is designed to compromise between a tucked and less aggressive/more upright position so it's better for street while still fitting in for track duty. The RS-1 is a little lighter and also it's a bit more quiet. Both interiors are one of the plushest out there. I've really only run top of the line helmets from Arai (Quantum F and RX-7) and Shoei (RF-1000 and X-Eleven) and IMO Bell beats them both.
"An armed society is a polite society when a man may have to back his last words with gunplay."
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What is my joy if all hands, even the unclean, can reach into it? What is my wisdom, if even the fools can dictate to me? What is my freedom, if all creatures, even the botched and impotent, are my masters? What is my life, if I am but to bow, to agree and to obey?
-- Ayn Rand, Anthem (Chapter 11)
A couple factors. The new 20s isn't out yet and IMO doesn't offer enough improvement to justify the cost. I currently have a Scala Q2 Pro that I've used for 2 years and I'm not terribly impressed with Scala's performance and features. Everyone that I've seen that has switched from Scala to Sena has been very happy. I think Sena offers more for your money and has a greater quality over Scala.
Lastly I was able to work out a deal on the dual pack ($30-40 cheaper then anyone has them for) by doing a bulk purchase of them.