I would just get the normal Scholastic American printings. (If you live in America, that is.) And for authenticity's sake, you could get the Philosopher's Stone instead of the Sorcerer's Stone. If you live elsewhere, I don't know what to say, just get the books. I mean, if you really like audiobooks a lot more, you could go with those, but I would recommend actually reading them at least your first time.
I have the Philosopher's stone but I own the older UK versions of the books, not the more up-to-date ones.
Okay, well, I'd say that's fine. I know there are a few minor problems that they fixed in later issues, but overall, if you already have the older ones, it's not a big deal.
What, like newer printings of the same books? I'm not really sure if that's worth the money.
Do they have the same words in them or are they different?
They might be slightly different, like if there was a typo they would fix it or a story error that was easily fixable they might do something. By that I mean that I know of one example where they changed something because it was a glaring mistake and easy to fix - when Harry and Voldy's wands connect in Goblet of Fire, shadows of all the people Voldy had smashed came out of his wand in reverse order. In the original printing, James came out before Lily, but we know he was smashed first, so in later printings, they switched it around and said Lily appeared first. So little things like that that are easy to fix, they would fix. But there aren't any huge significant differences, and if you already own a copy of each book you should just stick with the ones you have instead of buying a whole new set.
I have the Philosopher's stone but I own the older UK versions of the books, not the more up-to-date ones.
Okay, well, I'd say that's fine. I know there are a few minor problems that they fixed in later issues, but overall, if you already have the older ones, it's not a big deal.
I don't own Half Blood Prince and Deathly Hallows though...
Do they have the same words in them or are they different?
They might be slightly different, like if there was a typo they would fix it or a story error that was easily fixable they might do something. By that I mean that I know of one example where they changed something because it was a glaring mistake and easy to fix - when Harry and Voldy's wands connect in Goblet of Fire, shadows of all the people Voldy had smashed came out of his wand in reverse order. In the original printing, James came out before Lily, but we know he was smashed first, so in later printings, they switched it around and said Lily appeared first. So little things like that that are easy to fix, they would fix. But there aren't any huge significant differences, and if you already own a copy of each book you should just stick with the ones you have instead of buying a whole new set.
the designs are different basically 
(And Rexy even more, go Rexy all the way!!!!!)