Optimum hard drive setup

[DoNoHarm]

So i was thinking of swapping out my hard disk with a larger, faster one and using my existing hard drive as a scratch disk. However, asking around, some people are saying that it's best to leave the existing hard drive inside my computer and use the faster drive as a scratch disk. Does anyone know what the ideal setup should be? It seems crazy to get a brand new, larger hard drive but only use it as a scratch disk....