If you're asking that question, chances are that they won't actually be very noticeable!
In average use you probably won't notice anything at all. In Cinebench you will see a dramatic difference. (Presumably 50% or 100% depending on whether you're talking about 13" or 15") But in real world apps, even for those that take advantage of multithreading, most of the time you spend in the app is probably not going to be any different at all. You'll only notice the difference when you do anything computational. And chances are, that it uses the GPU anyway, so you still may not notice much difference.
A few years ago, my laptop was a MacBook Air 4Gb (2-core) and my desktop was a 5820K 32Gb (6-core). I was repeatedly surprised with how productive I was on the MBA (for coding), even though the desktop system was probably something like 4X faster for multithreaded tasks. So while we all keep wanting the top specs for everything, the actual impact on productivity is likely very small. And if you're in the exception, you know it.