It is evident that Marius only fell in love with Cosette's because of her looks. Logically, in the long run Marius and Cosette will realise they barely know each other and will probably live a struggled marriage. Eponine has an understanding of Marius being friends, and have the same passionate spirit. While, Cosette is weak.
I love Eponine and I felt really sorry for her that she loved someone who didn't love her back let alone oven notice that she was in love with him, but I think Marius was better off with Cosette. One reason is because she's of higher status than Eponine. Had Marius been with Eponine, he probably would have been looked down upon by society for marrying an impoverished "street rat" another reason is because I just like he and Cosette better. They fit better than Marius and Eponine do. I think the two were better off as just friends.
I think Eponine should have stopped waiting on Marius in hope that he would love him, moved on and found someone who would actually have loved her. She probably would not have died if she had done so.