Availability: In stock.
The Fresh Roast SR 500 uses the body of the SR 300 and adds in some much appreciated controls: a simple mechanism for setting low, medium or high heat, and a rheostat for adjusting fan speed (+/- 10%), as well as a digital timer.
I like the general design concept because it doesn't try to automate anything, but allows direct user control.
In terms of price and control over the roast, the SR500 is in line with the discontinued Hearthware iRoast2 but the SR500 has a more intuitive interface and a quieter fan. The SR500 does a much better job of agitating the beans than the SR300 - see the details page for a discussion of these issues.
I used the highest fan speed and got an acceptably even roast with the SR500. If you add a bit of stirring to the beans, or switching to cool to extend the roast, it becomes a great little roaster. In my opinion - the SR500 is a better roaster than the SR300, and worth the extra money.