BIG UPFRONT WARNING: All specifications of next weeks MBPs are RUMOURS. They might not be true. Even the release of MBPs at this next event is a RUMOUR, for all we know Apple might be releasing a new line of pet collars designed for AirTags. ..."unleashed"...
Now for some clarification: If you buy the Apple Silicon MBP with 32GB RAM, you are buying the MBP with 32GB GPU. Apple Silicon RAM is shared with the GPU, and there is no distinction between the two.
If you are asking if a 32 CORE GPU and 16GB of RAM would be better or worse than a 16 CORE GPU and 32GB of RAM, I don't know. I suspect more GPU cores would be of more benefit than more RAM, as 16GB is generally plenty. X-Plane 11 recommends 16GB, with 8GB 'minimum'.
The caveat here is that some of the RAM will be eaten up by the GPU, but as a 4GB GPU is 'recommended', 16GB should still be plenty to have it run just fine.
Getting the 32 core GPU with 32GB of RAM would be safest, especially for the future X-Plane 12 with unknown system requirements, but obviously more expensive.