I think it depends on the market really. Some countries one is more popular than the other. So it could be figuring out your target countries and which is more popular. But if it were me I would just use both.
The vertical to horizontal depends on your theme. There is a tutorial for converting to horizontal tabs. I would not use one that converts my theme, they are very badly done, they use jquery to replace the whole section and are pretty heavy on processing power.
Check this one as well, https://www.bellini-services.com/shop/front-office-features/32-extra-product-page-tabs.html You might contact him, because it does not say its compatible with thirty bees, but a lot of his modules are.
Everything should be compatible in the 1.0.x branch. There are only two module classes that are not compatible that we know of, caching modules and friendly url modules. Those are not compatible because we have added them to the core.
No, its not. I think he mentioned in another post about this module that it is using a 3rd party library for that functionality and it would need to be fixed if the library changes / fixes it.