They're both equally as responsive on mobile. If you have different grid needs, you can always define your by overriding their default theming.
Material UI is probably further along (read: less buggy, more performant) than Semantic UI, but SUI has a few more useful components that MUI doesn't offer (like an autocomplete API search).
Semantic UI is "easier to learn" because Material UI uses JSS/CSS-in-JS, requiring you to use theming HOCs. Meanwhile SUI uses straight up components and styles using CSS you import yourself, so you override the Semantic UI CSS framework. If you don't have a way to import CSS into your JS app already, you'll have to figure out that process (while MUI would work out of the box).
If you want to just create a portfolio website, and the goal isn't to pick up a component system, I would just stick to writing React + CSS/SASS/LESS (or try using a CSS-in-JS lib like Emotion/Styled Components). It allows you to really custom tailor your own design system and create something truly unique (rather than looking "bootstrappy" or like yet-another-Material-site).