This is probably gonna piss off some Reptile fans but here goes nothing.
Reptile's only redeeming trait is rooted in nostalgia. He was the first secret character and had cool moves from MK1-UMK3. Every appearance he's had since MK4 has been a disappointment for a lot of people. He had a cool moveset in MK9, but the general consensus is that for most of his career, Reptile looked like shit and had a terrible storyline.
Ermac, on the other hand, started out kinda "meh, another palette swap ninja." But he had really cool moves, and was an interesting character, even in the games that were poorly received. He has a huge fan base. I don't keep a tally on it, but at this point, I'm guessing he has a fan-base that surpasses (or at least matches) the amount of Reptile fans. I could be completely wrong but I read way more comments begging for Ermac than I do for Reptile.
It's a a really tough call. I honestly think they will both make the roster. I think that NRS is gonna give the people what they've been asking for (like they did with Smoke in MK9) and have Human Reptile. Since his lizard look in MK4 (and even more in MKDA... Yuck), fans have been awaiting his awesome, though somewhat boring, ninja look.
But I don't think it's gonna be straight up human. Maybe it will be similar to D'vorah, how she's totally an insect humanoid but with somewhat human colored flesh. We will probably see a scaly texture over a flesh colored skin with a slight yellowish/olive undertone. He may still have claws or spines.
If they go that route, I think they should ditch the green lizard head thing when his mask is off. It would be way less cheesy and ridiculous if when he pulled his mask off, he had a human flesh colored face while still having reptilian features; like an unhinged jaw, shallow nose, strong brows, long tongue that rips heads off, etc.).
Eh, I'm still gonna vote Ermac because I'm a biased asshole. Bye.