Graphical calcs are great for checking quadratics equations. Some now can transpose formula aswell as giving the working out. No good in an exam but great when you're learning on your own.
Both the calculators you mention are the ones that the my son's school recommend and sell.
Only really need them for maths and physics exams, not much call for them in the others (cheeky)
I'd check with the teacher first what they recommend... they will be teaching them how to use their calculators properly in lessons, so it makes sense to use the same calculator as the rest of the class. I ended up buying the FX85 for GCSE and then the FX-991EX for AS. Both are excellent calculators but it's horses for courses.
I think BigAde is spot on. Unless there's going to be a disaster, my son will be taking at least 1 Mathematics A level together with other science based subjects so I went for the FX-991-S-UH