Question

Also, which one do you get in HG for the event? SS?

Answer 1

In heart gold, you get latios, in soul silver, latias. it's the opposite of the in game Lati@s so you can get both. They don't come with the items.

Answer 2

"From Saturday, July 31st 2010 to August 27th, you will be able to download the Enigma Stone item onto HeartGold & SoulSilver. This item allows you to go to Pewter Museum, where the item will be unlocked and you will receive the Soul Dew. However, a Latios or Latias will appear in Pewter City in HeartGold & SoulSilver respectively."

-Serebii.net