There have been other threads on this, and it might be worth a search, but I think follow-on milk is essentially a marketing ploy from manufacturers, who aren't allowed to advertise first milk, and so "created" follow-on milk. In other words, they're much of a muchness. I would do a search on here to see what threads you find - lots of much more knowledgeable people than me have posted on it.
With aptamil, the later follow on milks e.g. from 9 months, etc may have looked cheaper but you actually got less feeds out of them as the scoop was bigger. The aptamil tubs told you how many 200ml feeds you got out of a tub and you got less out of the later follow on milks than you did from the straight 6 month+ ones. It may or may not be the same with the Hipp organic milk. I just kept using the 6 month+ milk until ds started cows milk at 1 year.