Ok, I guess the twilight background it is.  See, 
the sunset one matches all the colors in sakura's 
kimono, and the twilight one matches all the 
colors in tomoyos.  The whole cel looks a 
little...blue with the blue matting and the blue 
background (since both then match tomoyo) but 
that doesn't make it look bad...just very very 
blue.  ^_^

And by the way, I've been matting my own cels for 
um... five months or so?  It's really easy and a 
lot cheaper buying pre-cut, and it matches the 
sizes better.  Just invest in a mat cutter, a 
long straight edge, and you're all set.  A single 
sheet of mat-board mats around 8 cels, so it's 
handy to trade pieces with friends so your color 
schemes don't get too monotonous.