If you're interested in getting all the Hearthstone Anniversary cosmetics, you should do it on an Alliance character. This is because the Stormwind Portals are spawning constantly compared to the other portals.
There are many new awesome cosmetics from a new Hearthstone Stone of the Hearth, to transmogs, toys and a new mount Compass Rose. All of these come from defeating the 3 Hearthstone bosses that come out of the Portals making them highly valued.
Edit #1: At one point, it seemed like this might be fixed. It doesn't appear to be with Stormwind Portals still spawning much more frequently than others.
Compared to the Valdrakken or Orgimmar Portal spawns, which seems to spawn every hour or every other hour, the Stormwind Portals seems to constantly spawn in random phases, usually leaving at least one up at all times in some phase. Searching the group finder on a Horde character, the Portal list is usually barren. However, the group finder for Portals on an Alliance character, the list usually has at least some entries and groups.
If you're looking to get all the cosmetics from the event and don't want to wait, hop on an Alliance character and head to Stormwind. Search LFG for some form of «Stormwind Portal» groups and get farming! Make sure you tag Dr. Boom to get the loot.
Be warned though, there are many people who are trying to join these groups for the cosmetics and group leaders are getting spammed with lots invites immediately upon setting up a group.
It's unclear if Stormwind Portals are spawning too much or if the other portals aren't spawning enough.