What are some alternatives?
When comparing Affinity Designer and Sketch, you can also consider the following products
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Adobe Illustrator - Adobe Illustrator is a vector graphics editor.
Adobe InDesign - Adobe InDesign is a desktop publishing software application.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
GIMP - GIMP is a multiplatform photo manipulation tool.