As much of an amazing thing as slides were when they were introduced a couple decades ago, opposing slides are an equally large "game changer". You can always measure out sq ft and say one DP or 5th is bigger than the other but to me the opposing slides is what really makes the difference.
You can have a 100' X 8' rectangle but you're only able to be in one part at a time. A space that's more square will always feel roomier even if it's actually less total space. The bottom line to me is to get as many opposing slides as possible which can be accomplished with a DP or 5th. Of course, there are countless other pros/cons between the two as well.