Based on the actual in-game models on videos, their faces especially the eyes, are pretty much the same.
One can argue that they are totally different people, but one can also argue that they are the same despite the change in hairstyle and attire.
There is no confirmation as to who they actually are, so there's currently no right or wrong here.
This is so true. I also think it's too early to tell. And, I mean, these protagonists were designed for us to change later with customization too, so there is that to consider. In some ways, protagonists will never be designed in quite the same way they used to be. I personally think they look really similar, just different hair cut and clothes really, but we also don't have official art which would make all the difference. THAT could really help because the artist wild be able to add those little touches, changes in expressions, posing, etc. If they're meant to be different Characters. As it stands, we only have game footage, which is purposefully "bland" in expression so we can self insert. Looking similar doesn't quite guarantee they're the same though, as you said, because they aren't changing the game models (unlike gen 6 where ORAS did change the models). They could just look the same because they're a blank template and they don't want to redo all their customization art and coding. Easier to keep to the same models.
My personal theory is that they are the same protagonists technically (so we have the same mom), but in this alternate universe you actually grew up in Alola instead of just moving there, or at least have been living there for a year or more. I have zero evidence for this, it's just a guess, so take it with a grain of salt! I'm just going by the two seconds of game footage of the open garage that looks lived in and not full of moving boxes, and the fact that after SUMO they don't have to re-introduce us to the entire culture of Alola. A protagonist that just moved to Alola was a great lens and excuse to have all that exposition about trials, the Tapu's, etc. This time around we shouldn't need so much explanation cause we as the player have been with Alola for a year and know what trials and all that are, we have the anime now and all that.