Hrm.

Probably glass would be better... plexiglass will 
break down over time.

Two layers of UV-coated glass would provide twice 
the protection, but you'd have to take things 
like frame depth, heat buildup, etc, into 
account. And, of course, you mentioned the 
mats... so that's good. ^_^

The thing about UV coating is that you can get 
it "visible" (ie, the glass has a darkened tint) 
or "invisible". It's my understanding that 
the "invisible" kind of UV protection is more 
expensive... so you've suddenly doubled the cost 
of your glass. Then again, I haven't gone looking 
for UV-protected glass in ages, so I can't 
remember if the visible coating is so darkly 
tinted that it would impede a good viewing of the 
cel, or what. ^_^ You could probably stop by a 
local art framer and see if they have any 
examples to compare.

Good luck!
-Cres