Praveen, I’m guessing your main need for this lens is wildlife and bird photography. If that’s the case, then that extra 100mm or 200mm will always come in handy. You’ll find yourself using 600mm way more than 200mm or 100mm.
I’ve been using the Sony 200-600 for over 3 years now, and before that, I used the Sigma 150-600 for about 1.5 years. The biggest limitation I faced with the Sigma was that Sony bodies cap the FPS at 11 when using any third-party lens. So even if you’re shooting with a Sony body that supports 30fps, you’ll only get 11fps with the Sigma. That makes it tough to capture action shots.
The second issue I had was autofocus hunting — Sigma lenses are noticeably slower to lock focus compared to the Sony 200-600. And lastly, I personally didn’t like the color rendering from the Sigma when shooting RAW using the in-camera Standard profile.
I’ve been really happy with the quality of the Sony 200-600 — it’s been rock solid for me over the years.