It's really hard to choose between both of them. From what I had read Stemesh need to eat living people for using his more powerful abilities also he is bad at full contact fights. Other wise, Background has only one weakness - his left eye and also he is physical type.

They are in place where is hard to find people to eat (they are hobos, but I don't see them on picture or text), so Stemesh is out of options for gaining enough power to beat Background easily.

Also this alley dosn't have to much space so background can't just disappear in area and find way to make attack from left or right, ect.

To be honest I think that Background will be winner. Stemesh have problem in close combat and there is not to much of option to move. Background can easily take speed and be like human bulldozer running in to Stemesh. It's more than 180kg running in to you. Try to shot at boar running in to you at full speed, when you can jump to left or right side.

Stemesh can use eventually his ability to bond universe but still I don't know for sure how this can end up, and what this skill can make, it have to many options.

Still for using this powers he need calories, even if he have something he will use bond ability and what? After short time everything buck up to where it was. Maybe he changed position of Background and he showed behind him, avoiding first attack. But Background can be like bull turning again to face Stemesh again.

I tried to explain how I understand situation and both character ability. I hope I didin't make it too confusing.
But I think Background will be winner here.