Under-construction or ready-to-move in Sector 78?

Okay I'll be honest, I'm a bit stressed deciding. Just moved into TGB Group society last month, but my cousin wants to buy. He's torn between a cheaper, under-construction project (less EMI now) and a ready-to-move flat (paying more upfront). RERA is there, still scared of delays. My possession was in March, thankfully smooth. Which is better for peace of mind and long-term ROI? What does the community think?