While they both look fantastic, I like the metal flooring better. I think it does a better job at complementing the model.
I'd also keep in mind what terrain pieces will accompany your army. As well as what theme and setting fits your fancy.
You could combine the two an industrial lava base display so it looks like that scene out of star wars
Why not change the lava to something mor sci fi like toxic waste/acid then you could combine the metal flooring with the nice lava bases.
Late to the party - zero internets... I prefer the metal for sure, much more suitable to the 40k aesthetic I feel...