Each of these cars is very different, so I think you will have to decide what size and type of car you want first. For example, the S90 is an older design, has rear-wheel drive and a superb I6 engine mounted fore and aft. Fewer were sold and there are not as many custom or performance suppliers as for the S70/V70. The S70 is front wheel drive and has a sideways I5 engine, so it handles and feels different than the S90. The S40 is a much smaller car - again a different character and feel. I suggest you also look at the April Consumers' Report magazine to check the dependability tables. Good luck with your choice.