I agree with you. The chemistry between Seiya and Usagi is much clearer than between her and Darien. I was never completely sold on those two as Darien comes across as rather cold and I just couldn't feel much passion between them. With Seiya, it's definitely there.
Btw, you picture is lovely ^_^ I like your choice of background for the picture and you really make out the emotional conflict going on in Usagi's facial expression. Nice work.