Advice on Last Minute Pacific Islands Trip from SYD

Go with Fiji. I don't see why Samoa would be any hotter than Fiji. Costs are similar. Food is much better in Fiji. People are much friendlier and less overtly money-hungry in Fiji. Beaches and snorkelling are better in Fiji. Waterfalls and volcanic scenery are better in Samoa.

There are better options than Bounty Island, if you have enough time. Generally speaking, the further you get from the mainland, the better. Exactly where is best depends on time and budget.

Thanks for your advice tch7.
Fiji appears to be 24C at most whereas Samoa is about 28C.
24C just doesn't seem warm enough to lie on a beach chair - depends on ocean breezes a bit and personal preferences

I second tch's points and recommend Fiji over Samoa. And I actually thought Bounty was the nicest of the small resort islands close to Nadi. Of course you must spend some time on mainland Viti Levu for any non-beach-related activity.
And I can't imagine Fiji being too cold for comfort at any time of the year. Water temperatures also remain comfortable year-round.

a bit and personal preferences

I spent time on Fiji in August and most of the time it felt above 24C. It was certainly warm enough for shorts and t-shirt all the time. I don't like sitting in the sun, preferring the shade of a tree and a hammock. In fact, if the sun was out at midday it felt almost too hot to sit in the sun, let alone for activities such as hiking.

Cloudiness and sea breeze makes it cooler, and evenings could require warmer clothing as well.

I have just returned from a month in Fiji and a month in Samoa and Samoa is warmer than Fiji, including the water temperature but in both countries the North was warmer than the South, the wind and cold predominantly comes from the South East, it seems most of the Southern Hemisphere is getting exceptionally cold weather this winter.
Fiji better caters for the mid range traveller and I enjoyed the facilities but often had to brace myself when jumping in for a swim ( both sea and pool) but the actual snorkelling was generally better than in Samoa, it was comfortable to walk around in Fiji whereas in Samoa you want to do any walking early and be swimming by lunch time( it gets 28-30 degrees by about 10 am in the North)
If heading to Fiji look at places in the North like The Yasawas, Mamanucas, or around Nananu i ra . The weather was dryer and warmer - about 16-20 overnight and 22-28 during the day
I loved Leleuvia Island Resort for the beach, snorkelling, facilities but eventually the strong wind and cold got to me.Didnt even stop along the Coral Coast it looked wet, windy and miserable.
On Samoa it was the same along the South Coast of Upolu, too cold to swim and strong winds...people said it had been like that for days...weeks
I only stayed two days then went up to the North of Savaii and jumped in the water at Manase
28-30 degrees during the day , water about 22-26 , night time didn't get below about 22....Fales were basic, food traditional ( ie bit limited) snorkelling ok ....but the water was nice and warm and out side sunny and calm.....
I tried a few other places but kept going back to the North.
This winter is very cold and wet and windy...I have been in Fiji and Samoa this time of the year before and the cold and wind was never that bad - Of course the winter is starting to improve ?.. It will be better in a few weeks .....Well that's what I tell myself now I am back freezing in NZ and wishing I was still swimming and enjoying the sun at Manase.

Thanks everyone for the responses. We decided on Fiji mostly because of the shorter flights.
Went to Flight Centre to see if they would beat airfares and got totally misled into a hotel booking but not being told that most of the resorts in the Mamanucas have a mandatory meal plan. My fault for not doing more research and thinking Flight Centre actually cared. Cancelled the original hotel (Bounty) and researching other options.

If anyone has a good recommendation, would love to hear it. Now leaning towards Beachcomber..
Yasawas seem to far for the short time we have.

Exactly how much time do you have?

Few people do Beachcomber for more than a night or two - it's just a tiny little island. If you have more than 1-2 days, it's totally worthing going further, say to somewhere like Octopus.

Thanks again tch7, I was just reading about Octopus and it's at the top of my list.
As I have now discovered, Yasawas not that far, I swear I had read 5 hour ferry somewhere.

Octopus also has a resort boat that can pick you up from the mainland and might save you a night in Nadi.
The dorm there is very nice, one of the best dorms I ever stayed in; a good choice if on a budget. They also offer garden bungalows (shared facilities). There is a meal plan (as there is nowhere else to eat), but the food is good, as is the snorkelling on the house reef.