Tonga or Samoa ?

My fiance and I are looking to head to either Tonga or Samoa in Sept/Oct this year (2008) as we have free return flights with Air NZ, however, are trying to decide between the two.

We are only likely to have 8 or so days away, and are looking for a combination of relaxing at beautiful/secluded beaches as well as activities such as kayaking, game fishing and exploring nature/scenery.

We will probably opt for a mixture of cheap/luxury accommodation (as we cant afford luxury for the whole trip!).

Any information regarding which country is the best/cheapest/nicest/most beautiful would be really appreciated. They just seem so similar and it is really difficult for us to choose!

Thanks very much for your help.