At the end of the manga "Sundome", what happens to Kurumi Sahana? At the end of the manga, it was left unclear whether the main character had married Kurumi as she had blossomed into a woman, or whether he had married somebody else. Is that her, or did she die from her earlier illness? 

I know it was meant to be left as "up to the reader", but I would like to reach a consensus.

Personally, I like to think it is Kurumi, but I could understand any naysayers.
It is left up to the reader, but I think we can draw a conclusive answer from the hints given to us.

Theory #1: She doesn't die


The woman we see him with at the end could be Kurumi grown up. She is of the same build and may just have longer hair:
Even if it's not her, she could still be around - just not with the main character


Theory #2: She does die


Towards the end of the manga, she is very ill, more than we have seen in the previous chapters. There's also a lot of dialogue alluding to the fact that death may be near:




As you pointed out, the woman he is with at the end of the manga has a different personality from Kurumi, laughing at the Roman Club and the main character's request, as if she hadn't been around them in that time period:


The title of the manga, Sundome means stopping the moment before. This is significant as in the final few chapters, Kurumi admits to having a dream where she had sex with the main character - the one thing that the main character so desired, but was not allowed. It would be fitting if the manga 'stopped just before' they could do so.
When with the woman in the final chapters, the main characters says "You still exist", but, it sounds more like that character is remembering Kurumi as a fond memory. The orbs of light and the character's expression in the panel before the statement seem to indicate this.




Other information:


He is shown as becoming a resident doctor. He told Kurumi in earlier chapters that he would become one so that he could take care of her always. This doesn't indicate much either way about what happened, but it's interesting to take into consideration.


Anyway, there's no concrete proof either way, unless the mangaka reveals it. So everyone can believe what they want to believe :)