Humanity would win hands down because humans are war like race which had experianced war for much ot it's history ( Short Answer )
Now with Longer version.
It depends in what form there is contact if Equstria is found in a deep space exploration mission (with current technology using probes) with probe there won't be anything other then passive distance observation. I think it would unlike that even with ponies found by humanity as it would be too expensive and gain isn't that great. Most of what we could find would be civilization without advanced technology.
2nd scenario if there is portal or some shit like that on earth. Let's say it is some where in Central Europe. Then shit is far more interesting, as it again devietes. For simplicity let's say everyone knows about it and until it is determened who ponies are and there atitude to humans is, humnaity somewhat work together, like internation space station project. Humans would contact ponies if ponies don't immediatly are hostily to us and Equstrian magic isn't hazordous to humanity then, us humans would research ponies, magic, how magic works, how to counter it, how to weponise it. While doing that we would work in increase our influance over Equstria using manipulation and abusing ponie friendliness. Equstria would end up being splitted up to influance zone between nations and alliances, using our science pony magic would be nothing to us and couldn't harm us. After everything would be in place Entire Equstria would be half colony with Luna and Celestia and other strong individuals either killed or made so incapible of posing any threats to human interest that they coud be largly ingonred. And without bullets fired or many of them humany would make Equstria in to human colony.
Some of you will point out:
> Princensess would notice and fight back
no they wouldn't humannity has much experiance in warfare and subversion, with use of secret services (spies), special forces and other tools they would be elimited (not nesserly killed, just made incapible of threatening humanity's intrests)
> But magic
Humanity wouldn't go heads first invading Equstria or making coups. Magic would research to the point where we would probabbly know more about it then ponies themselves. And we would defintly find way to counter it.
> Dsicord and others
scinece of humans would find there weaknesses and eliminate them plius we would most likely also try to find way to manipulate them to serve our intrests
Some may want to picture humanity armies charging ponies and crushing there armies in matter of hours. Others would pciture Earth getting smashed by moon cause magic. But trully I think humans got it, we have way too much experiance in warfare , and that experiance would make use not our might of our tanks but rather our experiance of subversion, manipulation and adaption to take over Equstria. There woudln't be collum of tanks and planes going through portal to take over Eqsutria, it would be human diplomats and there spies which would do it and in end it is likely ponies themselves would become slaves to humanity