What are some alternatives?
When comparing Sunflow Rendering System and Indigo Renderer, you can also consider the following products
RenderMan - Advanced Technology from Pixar Animation Studios for Rendering VFX and Animation
Keyshot - KeyShot 3D rendering and animation software is the fastest, easiest way to create amazing, photographic visuals of your 3D models. Enter your search in the box aboveTryYour download of KeyShot is only moments away.
appleseed - appleseed is an open source, physically-based global illumination rendering engine primarily designed for animation and visual effects.
Mitsuba - Mitsuba is a research-oriented rendering system in the style of PBRT, from which it derives much inspiration.
POV-Ray - What was then called STAR-Light, and later renamed to PV-Ray and ultimately POV-Ray, has therefore been on this planet for a whopping 9, 190 (or 11, 000) revolutions by now (plus another guesstimated 150 above it).
LuxCoreRender - LuxCoreRender is a physically based and unbiased rendering engine.