Unfortunately there is no upgrade option from an existing Portal Engine setup to be upgraded to Kentico 12 MVC. As the models are completly different.
To answer your questions...
1) It really depends on the complexity of your site, how much customisation is done and also what features of Kentico you've used, as well as other things mentioned in the documentation.
I would recommend to look at the upgrade aspects of the documentation firstly for v10 to v11 and then v11 to v12.
Before each upgrade, it's important to check the release notes of each version as well, as it will help identify potential breaking changes
It's important to make sure you also get to the latest hotfix of the latest version
Remember to test each upgrade before proceeding to the next one
2) It really depends on your and your teams MVC expertise, but my recommendation would be to go for the MVC approach rather than upgrading.
It would also depend how you created and used the portal engine sites orginally. I say this because although we can't upgrade from portal engine to MVC, but say you created custom settings, custom tables, custom modules, page types that are content only etc... then you could look to utilise them in the new MVC build by bringing them over with exporting them and then importing them in. But it's important to bear in mind the different versions and if something has changed then it might involve some work.
There is also this blog post by Jeroen Furst, that could help with content migrating
I hope this helps