Bilsteins have their own warranty. As for resale after 3-4 years i don't think either of them will gain you much if any extra on a trade in. In fact most extras you put on a car don't, as the dealer will happily tell you it makes it more sellable but not worth any extra trading in, I found this out trading in my Beemer which had professional sat nav and elec leather heated seats, all dealers just quoted book value without the extras.