Regarding the code:

C++ is already commonly used cross platform for non UI code, there was a recent presentation on how dropbox accomplishes this:

http://oleb.net/blog/2014/05/how-dropbox-uses-cplusplus-cross-platform-development/

Since you intend on using C++, the non-UI code can be shared regardless of the UI decision. That might make it easier to just make a decision about the UI.

Regarding the UI:

You should determine whether this tradeoff is acceptable with additional research.

The last mile of making a hybrid mimic a native app is rarely worth putting effort into, as you will two problems: 1) finishing the app features, 2) hacking in all kinds of native functionality.

If you try apps made with mobile uis (this site[1]), apps made with mobile frameworks (ionic[2], ratchet[3], others...) and decide the performance is fine, then make a hybrid app. If you feel you need to improve on it, even incrementally, just shipp native controls instead.

Even if you end up intending on shipping native controls, you could prototype with HTML and later figure out the native UI, assuming that the non-UI code is decoupled.

[1] http://blog.stackoverflow.com/2011/07/mobile-stack-exchange/

[2] http://ionicframework.com/

[3] http://goratchet.com/

Regarding the code:

C++ is already commonly used cross platform for non UI code, there was a recent presentation on how dropbox accomplishes this:

http://oleb.net/blog/2014/05/how-dropbox-uses-cplusplus-cross-platform-development/

Since you intend on using C++, the non-UI code can be shared regardless of the UI decision. That might make it easier to just make a decision about the UI.

Regarding the UI:

You should determine whether this tradeoff is acceptable with additional research.

The last mile of making a hybrid app mimic a native app is rarely worth putting effort into, as you will two problems: 1) finishing the app features, 2) hacking in all kinds of native functionality.

If you try apps made with mobile uis (this site[1]), apps made with mobile frameworks (ionic[2], ratchet[3], others...) and decide the performance is fine, then make a hybrid app. If you feel you need to improve on it, even incrementally, just ship native controls instead.

Even if you end up intending on shipping native controls, you could prototype with HTML and later figure out the native UI, assuming that the non-UI code is decoupled.

[1] http://blog.stackoverflow.com/2011/07/mobile-stack-exchange/

[2] http://ionicframework.com/

[3] http://goratchet.com/