Re: Decking
We live on a very wet and windy site overlooking the Blessington reservoir. We got a tanalised softwood deck in 1996. The only treatment it has had since is pressure washing twice a year to remove the algae.
Other than the appearance which has aged, I would say it is as good as new.
The tanalising was done in Brooks Thomas and I seem to remember there was a fifteen year guarantee, so I would expect to get at least 20 years.
I imagine the lifespan of the hardwood deck is similar, but I think you have to oil the wood periodically. It's bound to look better when it ages, but is it worth the extra money? Probably not.
If you do go for softwood make sure the tanalising is done properly and get the end grains sealed.