Mainland versus outer islands

Hi,
I´m planning a trip to the South Pacific in 2014.
Will (after good advice here) spend most time in Melanesia.

But - as for Fiji, Samoa and Tonga... my question:
Should I definately head for some smaller island as opposed to the mainland?

To narrow this question:
In those 3 destinations - is any one of them OK just for staying a few days on the main island (where you land)?

All advice welcomed!
Erik

I think in Fiji the main island is beautiful and worth exploring.

In Tonga it is flat and not too scenic but offers lots to see.

In Samoa, there are no real outer islands anyway.
And of the 2 large ones, Upolou where you land is the prettier one.

Good advice!
Many thanks!

Samoa has no real outer islands, just two main ones. Fiji it is essential to get out of Nadi and head for the islands, they are pretty touristy but well worth it and its very easy with numerous travel agents eager to book you out there. Tonga it is very worthwhile to fly to other islands, the main island there isn't a great deal to see and do. Vava'u is probably the most popular destination by a long shot, although its less developed than Fiji and activities can very much be of the DIY variety.

All islands have much further away places, where there can be minimal tourist infrastructure and any activities you have to completely organise yourself. Such places take more time and money to get to, and you may have to rough it. Thats essentially the nuts and bolts of it.