in case you haven't read this page yet,
Choosing a Computing Option shows a good comparison. App Engine Flex and Compute Engine have some overlapping areas where either of them would be a reasonable choice, but I would think that if you put a weighted requirements check-list beside both of them you probably will strongly favor one over the other. The quotas/limits & pricing pages should also be of interest. I would recommend to go through some of their quickstarts very early, or use the interactive tutorial in Cloud Console / Cloud Shell (no local setup needed at this point).