It's not 'rich', and the calorie content is the same as ordinary formula. The protein content is predominantly casein, though (as opposed to whey) and casein proteins take longer to digest than whey proteins as the infant digestion system finds them harder work to break down. (This is not by the way, follow on milk, which is yet another formula, for babies over 6 mths, and which certainly is a marketing ploy).
All formulas used to be casein-dominant, until about (I think) 25-30 years ago, and whey-dominant was brought in as 'more modified' and closer to the casein-whey ratio of human milk.
There's no research as far as I know that shows babies fed on 'hungrier' baby milk do go for longer between feeds. If they do, they presumably need larger bottles as the calorie content of both types of formula are the same, so a baby feeding (say) 8 times a day on whey-dominant will need larger volumes if he is only feeding 6 times a day with casein-dominant.
It's all very theoretical - no long term research has been done on babies fed on either formulation to see if there are 'better' outcomes with the newer, whey-based milks, and so no real justification for telling mothers these are 'better'.
So in the end, I would say mothers who are not breastfeeding should try both sorts and see which one their baby seems to prefer.
Isn't it daft that this sort of information is not easily obtained? You'd almost think manufactuerers wanted to keep mothers under-informed....