Hi Zoidy,
While I'm slightly biased (I sell Monarch Spas in the UK), I can wholeheartedly reccomend a Monarch Spa from Spa World.
Check out their website:
http://www.spaworld.com.auMonarch Spas are without doubt the number one brand in Australia, and are sold through Australia's largest spa retail chain: Spa World, with 50 stores in Australia alone. They are also one of just a handful of AU manufacturers to have been in business longer than their warranty period, and have been producing Spas for 25 years.
The main things to consider when you get conflicting advice from sales people is to get them to back it up: ie: If someone says that their spa is stronger, compare the stuctural warranty to another brand. Monarch Spas use a timber frame, and carry a 10yr structural warranty. They also carry a 3year pump warranty which includes the circ pump. Most other brands offer just 12 months on their circ pumps.
Monarch use wood for their spas, and Galavanised steel for their Swim Spas. Steel is stronger, but does rattle. Australian houses are made from both wood, and steel. Yet they both stay upright!
More important than the frame material is the strength of the shell - Monarch shells are self-supporting which means that they are incredibly strong, and dont need additional support from steel or other materials. Steel is common on unbranded chinese spas found on eBay, as it us used to prop-up the shells.
I know which one I'd choose, but as I say I'm biased!
In summary - the warranty offered should indicate how weak/strong a spa is.
Good luck with your purchase.
Ed.