I use both in my introduction. I’m not an experienced author, but I want to share my thoughts anyway.
I begin my story with a short flashback, return to the present briefly, and then the rest of the chapter was MC reliving the past through a frame story (which contains most of the customization choices).
I began with the flashback because I first wanted to establish the MC’s voice (especially because I’m writing in first person and the MC was just 5 back then), and not so much the actual backstory. It’s more about the relationships with their family and what the effects the event had on the MC, than what actually happened. So when the story returns to the present, we already have a good idea about their motivation.
And yup, I agree with dwsnee above, you definitely don’t have to tell the flashback play by play (especially if it’s not something you want to remember!). I just recently realized too that my MC was actually an unreliable narrator in their flashback.
If what actually happened is more important, then it might be better for the reader to experience it with the MC, which is more engaging than a flashback. It can be emotional too, but IMO it needs a longer setup. If the event is a chapter long and not just a single scene, then the active event is definitely better.
It’s always a good idea to keep the customization choices in flow with the story, but unless it’s very important, you can afford to wait a little. Oh, and try not to lump them all together. Bonus points if the choice performs double duty. You can compare the MC’s hair with another character, but even better if it also describes their personality/relationship at the same time.
Welp, that became a lot longer than I thought. If you still can’t decide, just write and see if it works! I’ve lost count on how many times I’ve rewritten my opening scene. It’s all part of the process. Good luck with your project!