Blizzard Beach IMO. Better slides and better theming. But if your looking for more of a resort experience, you would probably enjoy Typhoon Lagoon more.
BTW I know the theming is better at TDS but like a said before it's not just about theming for me. I prefer the types of rides IOA has over the ones at TDS.
I'm going to Luna Park during Spring Break. Does anyone know if it will be really crowded and if certain days will be better than others? Also, is Thunderbolt as rough as everyone says it is?