Should I use Iris or OptiFine for Optimum Realism

Short answer: Use Iris + Sodium unless you have a specific reason not to. It runs the same shaders as OptiFine (Kappa, SEUS PTGI, Sundial, BSL, Complementary) but on a much faster rendering engine.

Why Iris wins on FPS: Sodium rewrites Minecraft's rendering engine from scratch and runs underneath Iris. The community typically sees 2-5× FPS over vanilla and 1.5-2.5× over OptiFine. Iris itself updates within days of a new Minecraft release — OptiFine often takes weeks.

When OptiFine still makes sense:

• You want a single mod that does everything (shaders + zoom + CTM + CEM + custom colours all in one)
• You're on a Forge modpack (Iris is Fabric-only — but Oculus is the Iris fork for Forge if you need shaders there)
• You've already set everything up on OptiFine and it works for you

If you switch to Iris + Sodium, you'll also need:

• Continuity — connected textures (Iris doesn't bundle this, OptiFine does)
• Polytone — biome / variant block colours (see why)
• EMF + ETF — custom mob models (see why)
• Fabric API — required by all Fabric mods

That sounds like more mods but each one is tiny (under 1 MB) and they're all maintained by active community devs. Drop them in .minecraft/mods/ and you're done.

For the full mod list including performance additions like Lithium and FerriteCore, see the Iris+Fabric mod list.