Only other thing to watch is the max transom weight. (and remember to add any Aux you might want)
Mine for example is rated for 60Hp. Problem is if I hang a random 4- stroke 60 on the back It will likely exceed the 110Kg weight limit...... Subtracting approx 10-15 Kg for an aux limits the choice of main engine somewhat!
And that, in a nutshell, is why I have a floating museum!
As you are probably aware, most outboards are built in groups of two or three. (e.g the Suz DT25/ 30, Merc Clamshell 50 / 60), so it's probably worth trying to get the catalougue for the 70 / 90 you are looking at - you may find for a given manufacturer they are the same physical lump so the 90 is a no brainer. Likewise comapring one brand to the other, for one the 70 might be the high Hp of it's 50-60-70Hp group, the other it's the low end of a 70-90-100Hp group, which means you end up lugging lots more Kg around for your HP.
If you are shopping around for a used engine you are more likely to be limited by what you can find, but it's food for thought.