I have a Pre-facelift E63s and have had many other AMG's over the past few years. From the small C63s to the over-the-top G63, I can for sure say that they are dramatic in their own way. The E63s seems to be the perfect blend of drama and comfort from the Mercedes cars while not being too overpriced. While I feel that the GT63s is the better car when it comes to dynamics and looks, I feel that the asking price doesn't justify it.
Going back to the main question, it honestly comes down to preference. Both brands have pros and cons to them, and when it comes to performance or comfort, the '21 version of both will be so similar that I don't feel like you could go wrong with either. I have an order in for a '21 M5 but that won't make my E63s any less valuable. I like both cars and their quirks, but I don't believe either one is a bad option. Just go with your gut and buy what you like. From what they offer, you'll get your bang for your buck regardless of choice.
I know this doesn't help much, but I hope this provides some insight, especially coming from someone who's always been an AMG guy
