I am not a coach so my understanding of a lot of the terminoloy thrown around (3 tech vs 5 tech, etc), is "ify' at best. I am sure my sense of the difference betwen a 5-2, a 4-3 and a 3-4 may be simplistic but I see it more as a variance is the position or pre-snap location of the front seven defenders primarily. Perhaps there are major differences in the technique or movements of each of the defenders depending on the offensive formation, the action of the offensive play as it develops and the responses of each defender thereto. What once were called 'rush ends' which I think of as the two outside defenders in a 5-2 alignment, become defensive ends in a 4-3 and I guess a 3-4 as well. My observation is that in most 4-3 alignments, you tend to have 2 bigger defensive tackles flanked by 2 outside 'rush ends that appear to be about half way between defensive tackles and typical linebackers. Of course, as with any position on either side of the line of scrimmage, the bigger and stronger and faster the better. But speed and agility and ability to tackle and get off blocks comes FIRST and you sacrifice size if need be to keep those other traits maximized.
DTs are typically employed to occupy as many of the offensive linemen as they can while attempting to push the line of scrimmage forward and clog up the middle and contain the outside as muc as possible. The fewer the number of big strong DT types the better they must be. Without the big guys dominating up front, most team will simply overpower using big RBs and a powerful O line to push their way down the field. As most coaches say, conentional wisdom guided by decades of experience is - stop the run first. If you can't, your almost doomed to lose the game. If you have a player or two of Suh capabilities, then you're going to have a great defense normally because the offense can't get enough one on one battles between blockers and tacklers, leaving unblocked LBs,. safeties, etc to crash in to disrupt, etc. So if you don't have "SUHs", then you better have great LBs and outstanding cover corners and great tackling safeties to do most of the tackling, etc.
Whether rush ends, LBs, defensive ends, or even secondary players come up near the line of scrimmage, or play back further, the defense is always looking for ways to avoid blockers and attack whomever has or gets the ball after the snap. In the 5-2, maintaining good 'gap control' to limit the holes in the defensive front line is critical because if a ball carrier finds a hole, there are not many guys between him and the endzone. I prefer the 4-3 alignment (as I said ' pre-snap positioning') as the comromise or best of both features of the 3-4 and 5-2. Unless you could have 2 Suhs, 2 John Duttons and 2 Grant Wistroms to rotate in and out to run the 3 man front. Ideally, you'd have 2 Wistroms and 2 Suhs. But if you have that many D linemen, you'd could run any of the fronts and dominate games consistantly against almost any team or any offensive scheme.
As they say, it's not the X & Os, its the Jimmys and Joes! I welcome any 'corrections' or suggestions on where I am wrong on the foregoing basics.