Macbook Air or a Windows laptop i5

For those of you who are getting quite confused with regards to choice of CPU, see below:

Well written explanation of how complicated a question it really is!

When you then start to consider Sketchup in all this, I wonder (as Sketchup apparently only runs on on core) if it is a logical decision to go for say a dual core rather than a four core (assuming all other specifications were the same)… or to put another way; when comparing similar speed/spec processors, would having more cores actually restrict the performance of Sketchup?..

Maybe someone with more than the limited knowledge of the current specs could confirm this?..