They need replacing so I want to do this job so it lasts for the longest time
If your open to alternative materials have a look at some of the composite decking available - they take much less maintenance (Enduradeck link). If your heart is set on wood, go for a hardwood but remember that even hardwoods require maintenance.
Need advice for my spare room
Need Advice on My Living Room's Paint Colour
Advice needed on my brown living room wall
Hardwood is still a great option and will last for decades if maintained. It would still be my first choice. In particular if it does suit the existing house.
The new composites like Millboard, Inex, both look good once down. The Inex for example gets stained (or painted) like a standard timber but the product is a bit like a fibre cement so is almost unaffected by moisture. How long will they last.... well they are quite new to the market.....