If I had to guess I would say that the Northern Hemisphere is preferential for seasons.The series heavily implies that Aang masters one element per planetary season, mastering water in the winter, earth in the spring, and fire in the summer, which is also kinda poetic as those seasons also correspond to their elements.
The wiki confirms that Aang awoke from the iceberg in the Fall of 99AG (99 years after the Air Nomad Genocide), but that doesn't actually tell us whose fall it is. The first major reference to the Winter Solstice comes in the two-parter "The Winter Solstice" while the gang is in Hei-Bai's forest, later journeying to the Crescent Island, both of which on the map of the World of Avatar appear to be almost exactly on the equator (perhaps a couple of degrees north of it), creating further ambiguities. It should be noted that it is only after this episode that Aang really starts to learn waterbending, as Katara doesn't steal the waterbending scroll until the episode following this. So I think it's safe to say that Aang learns waterbending in the Winter of the Northern Hemisphere.
The episode "The Avatar and the Firelord", set on the summer solstice, also begins at a spot very close to the equator, although perhaps slightly north of it: Roku's Island. In that episode, Aang had been traveling south from north of the equator to get there (as his last confirmed location was in the episode "The Beach", although the island's location in terms of equatorial position is very ambiguous. That said, when he recieved his vision from Avatar Roku making reference to the approaching summer solstice, he was certainly in the Northern Hemisphere.
When Aang starts learning firebending from Zuko, it is at the Western Air Temple, far north of the equator, supporting the theory that the northern hemisphere is preferential, if in fact he does indeed learn one element per calendar season as the creators have suggested.
In the series finale, set at the 'end of the summer', the group travels extensively around the world, probably more so than in any other single episode, but it begins on Ember Island, north of the equator.
It should also be noted that nights and days seem to be of equal length in both the northern and southern water tribes, regardless of the time of year, which is shown in Legend of Korra as well. Ba Sing Se is in the Northern Hemisphere as well, and really seems to be the center of the world in terms of diplomacy and global affairs, at least in the original series. The fire nation capital is pretty much on the equator, and the Air Nomads and Water Tribes both have equal land in the northern and southern hemispheres, but overall the World of Avatar, like our world, seems to have more land in the Northern Hemisphere. So that's what I'm gonna go with. The solstice episodes both happen in what we can presume is the Northern Hemisphere, as do most episodes which make reference to the season, and that hemisphere seems more central to the world, with Ba Sing Se and the more important and influential water tribe both north of the equator.