As someone with no greater loyalty to the '99ers than the '15ers or vice-versa, as a matter of honesty, I would have to say that if you matched each's best game against the other's, the '15ers win easily. The '99ers never put together a game in the '99 WWC like the Japan final. The closest was Nigeria, who was not nearly the squad then that Japan was in 2015. Over the course of the tournament, though, if one imagines the '99ers at mid-level play (say, the Germany game) against the '15ers in each of the '15ers' games, the '99ers more than likely win the Sweden, Nigeria and Columbia games, the '15ers definitely win the China, Germany and Japan games, and Australia is close. The Australia goals came up off left side play, and Joy Fawcett was a nearly flawless defender there back then. If the '15ers mid-level game (somewhere between Australia and China, closer to China) is played against the six '99 games, the 'analysis is harder. Group play was inferior in '99 and the subs played the North Korea game. The '99ers probably win the Germany game, the '15ers probably win the Brazil game (unless Scurry replicates her legendary saves) and the '15ers probably win the China game.
I think it's rather complicated and unwieldy to analyze who would win what against opponents from back then. I just believe the game has moved on in terms of not only depth but the overall skill level and athleticism. This is coming from someone that set my VCR to record overnight matches broadcast on Prime Ticket from China in 1991. I'm not so nostalgic that I lose my perspective. It's been long enough now to consider it a different generation if not a different era altogether. So for me the argument of who is best is a bit of an opaque question. If it's a question of who would beat who then I think the 2015 team would beat the 1999 team. If it's a matter of which team was greater in terms of greatness it's a different question. Who was greater.... the Big Red Machine Cincinnati Reds of the 70s or the New York Yankees of the late 20s? Head to head the Yankees would get throttled but asking which team is greater in a more philosophical sense is a different question. You're really asking which team stacked up better against their peers. From solely a WWC performance perspective I would take the 2015 team. They improved as the tournament went on to dominate the run of play in the latter stages. The 1999 team was inconsistent and though they ultimately won they didn't exactly do so going away. I felt, and still feel, that they underperformed in terms of their quality of play relative to how they were capable of playing. They seemed to, at times, feel the unprecedented level of scrutiny and attention on them. They had a lot of great players but not necessarily all of them were at their performance peak/prime. I do feel that the 1999 World Cup was probably more important in terms of further rooting women's soccer into the national psyche. But it's too early to say really what impact the 2015 victory will have.
My goodness... this is such an irrelevant question. You cannot compare teams from different eras on the pitch. It's total, utter fantasy to try. What will be more important is how much each team did for the game. And without the 1999 team, the 2015 would not have existed. Maybe some of the players would have appeared -- but not that team. The 99 team created the 1st professional league in the US. The 2015 has been responsible for creating TWO leagues. How well they sustain the NWSL will be a key part of their legacy. It's about getting sponsors -- Season ticket holders -- better stadia. That is the game that still has to be won -- not some hypothetical game between two incarnations of the SAME TEAM.