I've been delving into football the last year because I joined my first online franchise. What people don't understand is that the 3-4 and 4-3 are BASE defenses (or formations) that are used against an offense fielding 2 wide receivers. Anything with more than 2 WR utilize a nickel defense (against 3 WR) or quarter/dime defense (against 4 WR).
In Madden, you want to use the defense that best utilizes your players. Unfortunately, the game does not reflect each defensive players position well, as some players are suited to be 3-4 Linebackers or 4-3 defensive ends.
The first number designates the number of linemen and second is the number of linebackers.
3-4 Formation:
The defense fields three GIANT men [One nose tackle (DT) and two DE (players that normally would be a tackle in a 4-3 defense)]. The linebackers are then separated by 2 OUTSIDE linebackers and 2 INSIDE linebackers. The backs are all the same in 3-4/4-3.
It's the defensive lines responsibility to stop the run, and occasionally sack the QB (ala Wilfork) if possible. You want dudes with high block shedding, because chances are they won't even be able to get around double teams fast enough.
Outside linebackers will be responsible for most of the blitzing. Being able to choose between two different sides to blitz gives the defense a huge element of surprise. The middle linebackers are responsible for coverage and stopping the run, so they need to be the smartest and most perceptive players on the team.
ADVANTAGES: Able to disguise blitzes much better than 4-3. More specialized roles for linemen and d-line. Better coverage with 2 inside linebackers.
Disadvantages: (Historically) weak against the run, but Madden doesn't seem to reflect this as well.
USE 3-4 If you have a player(s) that fit these roles well:
OLB: High speed, accel, tackling, power/finesse move. These guys need to be able to sack QB efficiently. They should be freakishly athletic. Coverage skills is a plus.
ILB: High awareness, play recog, block shedding, zone/man defense, tackling. They rarely blitz, but are able to come through gaps in pass plays if necessary.
DE/T (3 in total): Your tackle should be your strongest, biggest dude and the other DE should be big as well. Focus on block shedding and strength. Pass rushing is a plus. They rarely move into coverage so not very necessary.
4-3
4 men on the line, 3 linebackers (two outside, one middle).
Strengths: You may never need to blitz, as your line will be responsible for all the pass rushing (and will be constantly pass rushing), while your linebackers are there for coverage/run stuffing.
Weaknesses: Somewhat weak against the pass over the middle. I have to constantly adjust to guard the inside when I play anything past cover 2.
LB: All of them should be coverage linebackers, good awareness, play recognition, block shedding, tackling, zone/man coverage. Blitzing attributes are a plus, but far from necessary.
DT/DE: They should all be blitzing machines. The LE seems to get most of the blitzes in Madden (as I imagine the best pass protectors are on the left side of the offense) so put your best pass rusher there. DTs need not worry about run blocking so much and need to take advantage of 1 on 1 situations.
If you have a tackle that has high power move, i'd opt to use 4-3. It's a game changing formation to use if JJ Watt or Marcell Dareus are only being blocked by one person. Also, I recommend this option if you don't have tackle depth, because 3-4 requires three big men (usually tackles) to utilize properly.
I am going back to work now.