Interesting Shellytt....
Well, if it was the first scenario, then Neville would have been the boy who lived. Judging from the bubble gum wrapper scene in OoP, Alice Longbottom loved her son very much, and would probably have shed her blood for him just like Lily did for Harry. Neville might not have grown up with his grandmother, because she was related to him from his father's side, and from Dumbledore we found out that Harry had to stay with the Dursleys because they were related to him through his mother's blood. So Neville would have had to live with someone blood related to his mother. Judging from the way they bring up Neville, his whole personality might have been different, depending on whether he was exposed to the Wizarding World during his childhood, the strictness with which he was treated, etc. He also might have been in a different house.
Yea, Harry would have had a much better childhood, growing up with the love he had always wanted in the original books. He might have had more siblings, and his personality too might have been different. He and Ron might have been friends earlier on, because I think Molly and Arthur were in the Order during the first war along with Lily and James. I'm not sure though...
I don't know about Snape. So many things could have happened. Like you said, he could have remained a death eater. But after Voldemort's fall, he also could have felt remorse that he chose the Dark Arts over his chidlhood friend, and attempted to patch up his and Lily's old friendship. He and James had too much enmity to become friends, but an air of civility might have been created. Who knows? Though maybe I'm just typing down something that is too impossible....
Also, Snape might have been put in Azkaban if you think about it. Because in one of the books, it said the Death Eaters from the first war were rounded up after Voldemort's "death", and most of them were put in Azkaban, except the few (like the Malfoys) who claimed they were under the Imperius. It seems like Snape would have too much pride to say he was under the Imperius curse. Don't know though.........
As for the second scenario, I'll give it more thought before writing a long reply like this....