Hi there

I'm travelling for the next 12 month on a RTW ticket, mostly with the following airlines:
Virgin Atlantic, Air New Zealand, Singapore airlines.
And now I'm not sure which frequent flyer programme I should join. What's the difference between the programme? It's all a bit confusing, help it's greatly appreciated.

Thanks
Katinka

p.s.: I'm already AA Advantage member (one world alliance), and I've found it very easy to collect miles with them and with their partner airlines.