Hi Marian, the both phones have the most same specifications, however in terms of aspect size and the resolution of the screen Oppo F9 succeeds. It has 6.3 inches (16 cm) compared to the Oppo F7 that has only 6.23 inches (15.82 cm). And to add up, the screen resolution for the F9 is 1080 x 2340 pixels not just like the F7 that only has 1080 x 2280 pixels (not a very much difference). But everything in general much alike/same.
For me, better stick with the Oppo F7 (in a practical way), since F7 is a little cheaper than F9 (also with faily specifications). Or if you want, spend a little bit more time on the phone you are using right now. And in time, surely eventually Oppo will do Launch a new phone with a little bit high specifications then you can have that one. We do all know the trends on phones are very fast, so let’s not keep up with the pace. Manufacturers are the only one benefiting from it.