I've been into photography for almost 7 years now and have been using the Canon 550D all along. I love it and have recently purchased a 1.4 lens which makes my photos even better. However, with my camera being really old and wanting my work to look better and more professional, I'm thinking of selling my Canon 550D upgrading to either a Canon5D mark II, or going mirrorless. I'm torn between wanting something lighter and easier to use and carry around, and crazy image quality. 
Also, if I were to sell my used Canon 550D which works perfectly fine but looks used (a few scratches here and there), how much should I sell it for?

I appreciate all feedback and advice, thanks! :)