[yyuugg]

In American English "next best thing" means second best. It is used to suggest an alternative when you can't get what you want. "Don't have Coke, maybe get a Pepsi, it's the next best thing."

"Next big thing" means upcoming good thing.

[_heimdall]

I've seen it used both ways, "next best thing" is one of those phrases at least in American English that is highly context-dependent.

[lolinder]

Which still makes it a bad choice for marketing copy.

They want something that isn't likely to be misinterpreted, and I've never seen that phrase used to mean "next big thing", only "second best".

[Zandikar]

As an American, I can't recall "The next best thing" ever meaning "second best' but rather the up and coming latest and greatest thing.

Big country though, could be a regional thing.