I am honestly considering getting PES 2018 this year. I'm kinda bored with FIFA. and the demo feels so weird to me. Is the master league better than the manager mode of fifa? This is the mode I play the most.
Master league is like manager career in FIFA. They are pretty similar. You can manage a club and international team simultaneously in both.
PES has more options for player roles and tactics, or at least those roles appear to have more effect than in FIFA. I find this part to be much better in PES.
PES is improving it's career presentation this year, but it probably won't look as flashy as FIFA.
Scouting is less deep than PES, but I prefer it. FIFA career requires you to either spend ages looking at dozens of scout reports, or just look up player stats on the internet (there's a reason sofifa.com is so popular).
In PES, you can search the player database and get accurate player stats, and your scouts make a few suggestions where they find players at a discount. Scouting is balanced by only allowing you to actively negotiate 5 deals at a time. It's less realistic, but I'd rather play the football game than bury myself in scouting reports.
PES fully separates wage and transfer budgets, rather than the infinite readjustment in FIFA. This can make it hard for boosting a low-league team, since the wage budget goes up slowly. You might sell some guy for 50 million, but you can't just put that cash into wages. This helps avoid the FIFA problem of everyone just buying pre-contract players for 'free' and selling them ASAP for quick cash (you can still do pre-contracts and free agents in PES, but you can't assign all your budget to wages to get Messi on contract expiry)
PES doesn't have that minigame training stuff. You set training styles (goal poacher, target man, etc.) for each player and the appropriate stats go up slowly. This also affects their AI 'personality' in game. You can also can give one player some extra training to get a special skill (some skill move or a bonus like 'captaincy')
TL;DR - If you love the scouting system in FIFA, stick with it. If you like the squad tactics side of management then definitely PES.