I would suggest....both!:P
No, I would say it depends what pokemon you like...
Ferocious beasts of Arcanine and Ho-oh?(HeartGold)
Graceful creatures of Ninetales and Lugia?(SoulSilver).
They're both equally good. Personally, I went on the boxart, and, well...Ho-oh scares me.
So if you get HeartGold, I'll be in touch offering hundreds of pokemon for your Kyogre:P