how to choose between CUBE and SPHERE

Hi

I'm making a virtual to that is composed of complete 360°, 360° cylindrical and partial (approx. 180°) panoramas.

I want to use HTML5. I have understood that HTML5 is only compatible with CUBE panorama.
So it's OK for the complete 360° panos.

But for the 360° cylindrical and the partials panos it sometimes make a CUBE and sometimes a SPHERE. Why ? How to force a CUBE ?

Edit :

- I use the vtour multires droplet.
- For the partial panos I edited my images to have a 360° panos, I simply painted the not used part in black

Regards

Julien