Neither sentence would be commonly used. Because of the uncommon construction, it's reasonable that there is a reason for using it: it's a rhetorical device.
The first sentence is an example of litotes, understatement. The literal meaning is that "this movie" is not as interesting as "that movie": the intended meaning is that it is far less interesting.
The second sentence could be taken the same way. There is a literal meaning, that "that movie" is almost, but not quite, as interesting as "this one". The intended ironic interpretation is probably that neither movie is particularly interesting.