In general, any laptop will run hotter than a desktop computer because it is more difficult to dissipate the heat from such a tightly-packed space. Putting a cooling pad under a laptop should help, since a lot of the heat is usually routed out the bottom. As laptops go, Apple significantly improved the cooling in the new 16" MacBookPro, so it should keep its cool better than other MacBooks.
More of both would help, particularly the graphics memory if you are maxing it out. Note, however, that Apple uses Radeon GPUs on many models, and some renderers (notably V-Ray) will resort to CPU-only rendering when they detect one. V-Ray will use GPU rendering only if it detects an NVidia GPU because its GPU hardware code relies on CUDA, which is not available on Radeon. So, if your chosen Mac has a Radeon GPU, maxing the main memory will be more important.
SketchUp itself uses only a single CPU core no matter how many your computer has. This is a fundamental limitation of 3D model editing not just of SketchUp. You can look up single-core benchmark results for the particular CPU in each Mac to see which is fastest. Don’t compare clock speeds. Internal details of the chip can easily make a bigger difference than raw clock. And ignore “turbo boost” clock speed. It only applies when the CPU isn’t working hard, ironically, when there is
already lots of headroom anyway.
My own research showed that there isn’t that big a single-core speed difference between current models of Intel processors. For example, the i7 in my MBP is only 8% slower than the alternative i9.
On the other hand, CPU renderers use as many cores as you have. So if you will do heavy renders, a CPU with more cores will be better even if its single-core speed is slower.
The main memory can impact performance while editing a model, particularly if you will be running things such as a renderer simultaneously. I believe the memory on an iMac can be upgraded, but I know that the memory on a MacBook of any variety cannot. Apple memory is expensive. So, for an iMac I’d suggest starting with perhaps 24-32GB and seeing if you saturate it. For a MacBook, you should opt for the most you can afford since that’s all you will ever get.
SketchUp itself doesn’t put too great a strain on the graphics. You won’t notice as much difference from this as from other aspects. That said, 2GB is somewhat small in today’s world. 4GB would be better. I don’t know if you would see any improvement with 8GB.
I don’t use Layout intensively enough to be able to say much on this. LayOut is written in a totally different way than SketchUp, though I understand it embeds a SketchUp engine for drawing into viewports. There have been numerous reports recently of Layout performance issues.