Why would you want the mental ray rendering engine? If the interface is 3d surely you would be loading the model and rendering it in your engine/game. So basically the decision needs to be made on modelling tools available and ease of use.

Mental Ray is a great renderer, and I can see it being used to do some prerendered 3D objects for the UI... that should then be manipulated with Photoshop or the like.

If you want a 3D UI that rotates or something like WC3, sure, you're probably just loading models to render at display time. If you want high-end 3D looking stuff (like some of Starcraft, say) that you're just going to use as a 2D UI, Mental Ray is great for lighting especially.

Still, though, it's about your budget and capability.