I find that my framed cels do much better simply 
because once they are framed they are subjected to 
less handling.  I framed this Sailor Moon cel, 
professionally with all the archival bells and whistles, 
because of all things the background.  Every time I 
looked another yellow smudge was on the cel bag.  
Toei means line fading but I really don’t see too much 
difference between then and now.  Even hung in the 
closet my cel books get handled, I move the cels 
around as my collection changes, I’m sure that that is 
worse than any exposure to the light.  The one thing I 
regret is that the cel is against the background but 
changing it would break the seal (yes the frame is 
sealed against air, supposedly to slow oxidation)  The 
guy who does my framing also does work for the 
Smithsonian, but he gives discounts when I bring in the 
videos so he can see the anime the cel is from!  This is 
a fun hobby.  Sadly I’m out of walls so I have cut way 
back on the number