In my opinion, Noida Extension is best for investment because:
1. Metro will be coming here in another 3-4 years
2. It is very close to Ghaziabad, Delhi and other parts of Noida
3. It will see great infrastructural development
4. All good builders have set up their projects here
5. There is no legal issue here now.
It depends on your budget and investment time frame. If your budget is small and you can wait for minimum 5 years then Greater Noida may be a good option. If your budget is good and you have a shorter time frame (2-3 years) then consider investing in Noida, as it is closer to Delhi.