Both resorts are right near each other, with The Sheraton Villas in between them. You can us the facilities at all 3 resorts.
Having stayed at both, heres my observations...the upgrade probably is worth it if you plan to be on your balcony looking at the view a lot. Snorkelling is not good, theres nothing to see, but the water is quite ok for swimming.
I prefer the rooms at the Sheraton, they are light & airy & a bit more spacious. The Westin is much more traditional Fiji decor, & the rooms are dark wood. However the Westin has the most wonderful beds & linens.
I prefer the breakfast at The Westin-the Sheraton is very noisy & not quite the variety of food.
Theres a general idea that theres less kids at The Westin,but I havent really found that to be the case.
The Westin Pools are nicer, lots more places to sit. We actually prefer the pool in the Sheraton Villas over all 3 properties pools.
I would prefer September, as November is closer to wet/cyclone season, but the weather can be crazy whtever time you go.
For what its worth, we are going back to The Westin in November ourselves on a similar deal to the Holiday Centre one you mentioned. We also did the same deal as you a couple of years ago & it was great.
I'm happy to help if you have any more questions.