I have a lightweight 2.7 rib, and 2 motors, an 8hp yamaha and a merc 3.3. the 8hp will happily plane the dinghy with 2 90kg adults. I use mine for the odd mission like setting a longline and the 8hp gives me the flexibility to get more places to do that.
I like the 8 and if i had to choose one or the other would pick that. But if the dinghy was heavier and it didn't plane with 2 adults then in practise it wouldn't really do anything that the 3.3 doesn't.
So my advice is
5hp if you use the dinghy a lot on your own and your dinghy is light enough to perform with you in it.
8hp if you use the dinghy a lot with 2 people (and you have a light enough dinghy that it performs with 2 people in it)
a 2 or 3hp if you don't use it much outside boat hopping/short trips ashore