You can see in the core. The class `\Magento\Catalog\Model\ProductRepository` implements interface `\Magento\Catalog\Api\ProductRepositoryInterface`. So basically, you can use the function `get($sku)` of both. The difference is you the `ProductRepositoryInterface` usually use with the API as you see. If you don't use the API, use the `ProductRepository`.

The ProductRepositoryInterface with no content in the method so you can find it in `var/generation/Magento/Catalog/Api/ProductRepositoryInterface`. Magento will auto generate the code.

There are what I find out! Hope it help!