The description about "Sturdy body" and "Highly curious" are basically personalities that tells you which of its IV is the best (IVs are basically the values which determines your Pokémon stats). So for "Sturdy body", it means that your Pokémon has a higher HP stat than its other stats while "Highly curious" tells you that your Pokémon have a higher Special Attack stat. Slyveon is a bulky Special sweeper, which really depends on which stat you prefer investing into Slyveon, for e.g. you want your Slyveon to be a Special sweeper that you should pick the "Highly curious" one. Personally, I would pick the "Sturdy body" one, since Special Attack does not really matter that much if your Slyveon is a Support.
Investing in HP is a good call as you want to maximise your bulk so as to take hits from threats like Mega Medicham or Mega Lopunny. And you shouldn't really invest in Defense since Slyveon is meant to be a Special Wall anyway. (Slyveon's Defense is terrible) So a spread of 240 HP / 252 Sp. Def / 16 Speed should suffice as you will want to put 16 EVs in Speed to outspeed uninvested base 60 Speed Pokemon such as Clefable, opposing Sylveon, and Tyranitar. I know I'm going a bit off-point but just consider my opinion.
