Don't want to compare the two, but in these mid-range smartphones, you have to do a compromise based on your preference and liking.
For Example, Oppo F15 has a MediaTek Helio processor while S1 Pro has Snapdragon (although a bit older one, still Snapdragon is much better in terms of efficiency and battery management), also F15 has a plastic back and a plastic frame, while S1 Pro has an Aluminium frame. Similarly, Bluetooth 4.2 in Oppo F15 and 5.0 in S1 Pro - a big difference in terms of connectivity and range.
But you will not get Gorilla Glass 5 in S1 Pro, very solid glass, but still not the Gorilla Glass 5 which is there is Oppo F15, similarly, gyro EIS in Oppo F15 and it's also lighter (172 g) and S1 Pro is 186.7 g.
The same is the case with Xiaomi Poco X3 and any other mid-range smartphone. The point which I want to make here is that these mid-range smartphones can't be compared with each other because everyone can choose according to one's preference about the features one like, like some like AMOLED screens, and others like faster processors and some like bigger batteries, etc.
So my suggestion before buying these mid-range smartphones is to choose according to your liking and be very specific about it as you have to do a compromise for the feature that you'll be getting in this price range by giving up the features that you can live without. That's the way things are with these mid-range smartphones, you can't get everything in this price range.
I hope it helps.
