Man to man or zonal defense?

[adifairus]

if your defender got higher marking, higher strength and almost equal speed attribute compare with opponent attacker, it is fine to use man to man,

but make sure you got enough man on your half field to mark opponent attacker.

if your defender got higher positioning and tackling attribute, it's good to play zonal marking, speed doesn't matter in this case.

but strength always a matter for your defender to win the ball

[Toxcatl]

if your defender got higher marking, higher strength and almost equal speed attribute compare with opponent attacker, it is fine to use man to man,

but make sure you got enough man on your half field to mark opponent attacker.

if your defender got higher positioning and tackling attribute, it's good to play zonal marking, speed doesn't matter in this case.

but strength always a matter for your defender to win the ball

I'm sorry, but it's just some speculations. There is no proof that skills do that, unfortunately.

Man-to-man increase your team's defense efforts (whatever that means) and also increase tiredness of your players. As has already been said, it should be used only if your defenders outnumber your opponent's attackers, otherwise it only makes things worse (AM will tell you that). High Pressing and Man-to-man each increase condition loss (CL) by ~30-35% if I remember Al's test results correctly.
If I need defense boost and my defenders outnumber their attackers by at least two, I use Man-to-man. Otherwise I use High Pressing. If I really need to hold for a few last minutes, then I use both. But I never do it in the beginning of the match, or players will be tired very quickly, and their performance in the 2nd half will suffer greatly; 160-170% CL is not a joke.