Sergachev has all the tools to become an elite defenseman at the NHL level, I would say that he possesses higher ceiling than Drouin, I can envision him becoming the complete package who excels in all three zones. At the opposite, mainly due to his size, Drouin is more restricted in a purely offensive role. Sergachev has the size, the speed, the shot, the IQ and you can add to that exceptional work ethic, he is anything you can desire in a NHL Dman. That being said, Dmen take longer to develop, so are harder to project, he is going to become a good NHL player for sure, however reaching the elite status never is a guarantee. From Drouin side, he isn't as complete as Sergachev could be, however he excels more in his strengths, his offensive IQ and puck skill are completely off the chart, very few player can create offense like Drouin can do, if he could only put up half the numbers he did in the QMJHL at the NHL level, Drouin would be one of the most productive forward in the NHL (Last season in the Q: Reg. season+Playoff+MCup: 66-43-115-158 -50%-> 66-22-57-79).
All in all, it seems like a pretty even deal that makes sense for both sides with each team addressing one of their biggest need, TB got a bit higher ceiling in Sergachev and MTL got a bit more guaranteed value in a older and more proven young NHL player. Like for any deal, only time will tell who got the short end of the stick.