For a serious long term project I'd certainly use 3.x, to be future proof. It'll probably take some sorting out compatible packages, but it prevents a technical

For a serious long term project I'd certainly use 3.x, to be future proof. It'll probably take some sorting out compatible packages, but it prevents a technical debt. Also the added performance, features and cleanups are worth it. For a quick hack I'd still use 2.7 because everything is compatible so it should be easier to get up and running.