Those are almost generic terms, it's more important to try and establish the species you're actually buying.
Redwood is a term used originally to describe a type of pine, usually scots pine. whitewood on the other hand, I *think*, can be and is used to describe anything from larch to spruce to pine and god knows what else. Like anything, your mileage may vary, same with decking. Decking can be £3/sqm or it can be £30+, the quality likely very different. Whitewood is rarely useful for much other than firewood. Redwood is usually much heavier to hold for a given length and has its uses.
For outside furniture, you can either go for pressure treated timber thats ready graded, say, C16, knowing that if you cut it you're going to have to re-treat it (not a great route as cutting has hazards and you sort of lose the effect of the pre-treatment), or, you can use something else and treat it yourself. There are a few hardwoods which are great for the outdoors, like oak or teak, but obviously the price rockets up compared to whitewood, as you'd expect.
There are tons of options, but generally speaking, the redwood and whitewood from B&Q and Wickes are absolutely awful, so your best bet would be to speak to your local timber merchants.
What do you have in mind to build? Knowing that will help people recommend a material.
It's not a stupid question at all, by the way.