The key point is that you (the player ) should decide and not the stringer or coach. By all means, be guided in the right direction but if finances permit try a few variations of string types & tensions to see what you like best.
In a nutshell:
1/ Full polyester for more spin and control but harder on the arm.
2/ Full soft strings for more feel and less loss of tension. Natural gut is the best but very expensive - only if you play in dry conditions and don't use too much spin as the strings won't last!
3/ A hybrid of polyester and a softer string which provides a mix of good spin, power, and feel
What do the top male players in the world use in terms of strings/tensions:
Novak Djokovic | Babolat VS Team Natural Gut / Luxilon ALU Power Rough 59/56lbs
Roger Federer | Wilson Natural Gut/Luxilon Big Banger Alu Power Rough 48.5/45lbs
Andy Murray | Luxilon Big Banger Alu Power/Babolat VS Touch 62lbs
Nick Kygrios |Yonex Poly Tour Pro 125 51lbs
Rafael Nadal | Babolat RPM Blast 130 55lbs
Gael Monfils | Luxilon Big Banger Alu Power 57/55lbs
Milos Raonic |Luxilon M2 44/46lb
Alexander Zverev | Babolat VS Touch/Head Hawk Touch 62lb
Roberto Bautista Agut | Luxilon Big Banger Original 57lb
Diego Schwartzman | Luxilon Big Banger Alu Power 50lbs
Out of this group of 10 players we have 6 using only polyester and the other 4 with a hybrid of polyester and a softer string. Tensions vary from 44/46lbs - Raonic to 62lbs - Zverev & Murray. Therefore, quite a variation in string types and combinations plus different ideas on tensions.
In general lower tensions provide more power and higher tensions more control. Polyester strings can be strung looser and still have decent control which is probably why all the players using polyester only are no higher than 55lbs - Nadal. (Rafa is one of the few players on tour who never changes his string tension no matter whatever the conditions are)
For young players and lower-level club players, a hybrid restring is generally better to help protect the arm and provide a bit more feel. If a mid-level or slightly older junior likes polyester I would advise no more than 50lbs again to protect the arm and provide easier power for slower racquet head speeds.
If you want a restring that doesn't lose tension quite so much then VS Gut would be the best but as I said previously it's very expensive and breaks easily.
Higher-level club players are probably the ones who will experiment a bit more to find out what suits them best and a huge consideration for aspiring young pros is the cost as they will break strings regularly no matter what the type of string they use!
To sum up... try not to get too fanatical about strings as a good stringer should be able to guide you towards the most appropriate tension and string type for your level of play. The higher the level of player the more they will feel subtle differences. You will sometimes see pro players complaining to their team about string tensions in practice or during matches which can be genuine but can also be players looking for excuses.
Find a string and tension that you like and stick to it - just like Rafa!