Once someone's past their first build, this is the next fork in the road. Both are good. They're just good at different things, and picking the wrong one is the most common expensive mistake I see.
The 5-inch is the default, and it deserves to be. It's the class with the most frames, the most spares, the most tuning knowledge online and the most forgiving crash behaviour. Four to six minutes of hard freestyle, sharp and responsive, cheap to repair when you inevitably bin it. If you want to learn, throw shapes and not cry over parts, build a 5.
A 7-inch is a cruiser, not a bigger 5. Bigger props move more air more slowly. It's smoother, quieter, floatier, and it'll stay up for ten to fifteen minutes depending on the pack. It covers ground beautifully. What it will not do is snap around like a 5 — the inertia is real, and tight proximity flying on a 7 is a genuinely different skill.
Flight time is the honest headline. If your flying is "get to a spot and explore for a while", a 7 changes the experience completely. If your flying is "five packs of gaps and dives in a car park", it's the wrong tool and you'll feel it in the first session.
Crash cost scales. A 7 carries more mass at more speed. When it goes in, it goes in harder — arms, props and camera lenses all cost more, and the frame takes real punishment. There's a reason people fly 7s over open ground and 5s over grass.
Where you fly should decide this. Chennai splits fairly neatly: tight spots, structures and parks reward a 5; open ground, coastline and long cruises reward a 7. Be honest about which describes your actual weekends, not the ones you imagine.
And on cost — a 7 is usually the pricier build. Bigger frame, bigger motors, and it wants a bigger pack to be worth flying at all. Batteries are the part people forget to budget for.
If you're only building one rig: build the 5. It's the better teacher and the cheaper mistake. Build the 7 second, once you know whether you're chasing tricks or chasing distance.
Not sure which your flying actually is? Tell me the spots you fly and what you want out of a session, and I'll spec it honestly — including telling you if the one you're leaning toward is the wrong call.