I'm getting a tenth the crown payout of FSC and no mats, and with my current completion time for it I don't think I'm getting any more rads per second. The speedruns I can google seem to reinforce this, FSC are lower.
How many more rads are you getting? Like 25% more rads? If I'm taking 25% more time it doesn't sound worth it even just for rads, but I've only run it a half dozen times I think and I don't know exactly how many rads the average is or how fast it can be done. The most rads I've got from FSC is 30 though. I'm yet to get that from D&N.
If you haven't already, you should check out the Lancer Knightz data sets --- particularly the one about Radiants in missions:
https://wiki.spiralknights.com/Lancer_Knightz_(Guild)/Mission_Rarities
To clarify, you're looking for the FC5 column. The data don't address time spent. I don't know about you, but Dreams and Nightmares takes me much less time than King of Ashes.