CUPID fellowship sounds like a sweet deal in general, however, I would suggest you go the clincal research route - for one purpose..
PUBLICATION OPPORTUNITIES!
Seems, like the CUPID fellowship doesnt have room for PUBS, but if it does then go for it. Then again if your clinical research route has no opportunities for PUBS, then CUPID sounds better overall.
If you have a chance, try to do clincal research in rad onc (duh right) at a place with a residency program so
#1 You can get your foot in the door and make a good impression
#2 You can get a LoR