The fact that it's a 240mm changes some things:
- not really that much different from the 210mm you have, unless you need the extra coverage for some reason
- rated coverage is slightly less for Sironar than Sironar-N
- I doubt you can get a 240mm Sironar-N in shutter for $200
- 240mm Sironar, Sironar-N, etc come in a #3 shutter. A #3 shutter is worth something by itself but it also implies big and heavy.
- I've only had one Sironar but I wouldn't get put off by putative optical quality problems, unless it already has rainbow separation which should discount the price. I find it actually quite hard to see an effect of rainbow separation in images unless you are shooting into the sun, but of course it has an effect on resale value.