Cook Islands or Samoa
 

Any opinion on which might be a better location? I like to relax on a nice, mellow beach, as well as snorkel and kayak.

Although I have been to Samoa several times and love the place, the beaches are not that terrific, well compared to Aussie beaches. A mellow beach, snorkeling etc. fine, but I think the Cooks are probably better for what you have in mind. Samoa, however is much easier to get to. Either place is the real deal South Pacific.

No comparison, go to the Cook Islands, much nicer.

downunder...can you tell me what to do in Samoa.....should I do an Aggie tour half day

Do the full day Aggie tour. They take you to the far side of the Island where you can swim and snorkel(or neither) also includes a BBQ lunch from Aggies kitchens. On the return you call into a coconut plantation for a husking demo. etc. It is a great way to see the best of Samoa. Other "musts" are Robert Louis Stevenson's house, Vailima and I would suggest taking a cab there as they have guides that provide excellent tours. The local market in the middle of town is definitely worth a visit and the Aggie Grey's Special Cocktail in the cocktail lounge is the best drink in the South Pacific!