I decided to watch them more... from what I've seen Semedo is the better defender, stronger, but also good in attack; crosses, link up play, very good passing..its all good. Pereira is lanky, agile and as a result seems to lack in the strength department which he makes up with his acceleration when defending, I think he edges him in pace though both have great pace, his passing and crossing are very good, he delivers potent sharp low crosses and he seems to utlize available space better than semedo, who I find is a little clumsy utitlizing space and runs into players often; Pereira is great at this, very clean, doesn't let players near him he knows he lacks strength so he always keeps distance, uses his agility, pace and finishes with great crosses. Dribbling I also like him more, like I said seems to be a cleaner dribbler.
Pereira edges him going forward, at least for my taste, whereas Semedo is the better rounded player overall. Both are acceptable choices, to say the least. Just my take so far.