Hmm, i think what you really wanted to ask is how railclone compares to displacement and not to PBR. I think it depends, but IMO railclone is superior in many ways. I don't know how they compare in RAM usage, but displacement is not cheap in that regard and on large scale scenes it can take take all the RAM you have and more. Railclone is much more flexible when it comes to adjusting the look and randomization. On the other hand, displacement can be more flexible in adjusting quality vs performance ratio - when you see that your scene is struggling to render, you can quickly increase displacement pixel size and get better performance, where with railclone it's more like on/off situation.
In Corona 9 you will have pretty worthy built-in railclone alternative - CoronaPattern, while it's nowhere near as flexible as former tool, it will give you huge performance boost, as its RAM footprint can be ridiculously low. You can already try it by downloading the latest Corona daily build and see if it works for you better than railclone.