@ninkinoarimasu, I will always fall on the side of integrating your splash screen into your Babylon scene because you have a lot of control over the behavior of the splash screen as well as using the splash screen to trigger events in your game or receive information back from your game.
While this example is not a splash screen, you can see that the playground starts with a UI button that disappears when pressed, causing the scene to start a particle system as well as some animations in node material parameters and vertex displacement. Then once it is all done, the UI is bought back to play the reverse of the initial states and back again.
This could easily be expanded out to a full splash screen with buttons or other information inside the splash screen. Things like high score, feedback on how the player did (points, combos, whatever), or really anything else you want to communicate to the player from the game loop.
Hope this helps!