I can't provide you with numbers but I can provide my experience from collecting during that time.
Everybody knew a big story was starting with MoS 18, I'm sure retailers ordered accordingly, but the cameo books completely took people by surprise. When people finally caught on to who it was in those cameos shots, those books quickly disappeared, with MoS 17 being the one highly sought after. It immediately jumped to be $15-$20 book. I can't give you the numbers, but you could find tons of MoS 18's but MoS 17's were very hard to come by. IMO MoS 17 from the very start has always been the more valuable book.
I know we like to place rules on these types of things but I think each case is unique. In this case, the cameo is harder to find. Plus MoS 18 isn't really much of a first appearance. He's hidden in a suit the entire issue. If you were to show the average person who knew what Doomsday looks like, but knew nothing about comics, the cover to MoS 18 they'd say... "That's not Doomsday!"
So call the issues whatever you like but at the end of day I think 17 is and always will be the more valuable of the two, though 18 will close the gap some. There is room for both to be worth owning. It doesn't have to be a war of one against the other.