When you have a crushed bent exhuast system you end up losing the over all diameter of the exhuast. Lets say you are getting a 2.5 inch crushed bent exhuast. Well the straight part is 2.5 inch but when it gets to the bend since it is crushed bent the diameter will drop to something like 2.0-2.25 inches. By laws of physics your exhuast system only flows as good as it's smallest point so while you may have a 2.5 inch exhaust system it is only flowing as good as about 2.0-2.25. Now mandrel bent exhuast system keeps the bends the same diameter so it stays a true 2.5 inches the whole way. This makes things flow better than the crushed bent. Now is it really that much better? Some debate on this and I'll tell you my opinion, take it how you want. On a turbo car it is well worth the extra money for the mandrel bent system. On a non turbo car it really depends on what car you are talking about. To simplify lets talk only about the 300zx. On the turbo yes and on the non turbo no. The non-turbo barely makes any extra hp from an upgraded exhuast that it really isn't worth the extra money becuase the difference between a crushed bent and a mandrel system is only about 1-2hp where as on a turbo it would be much more. However if you planned to really modify the non turbo 300zx, and I mean internal engine work, then I would say spring for the mandrel bent system. If you are just planning on some minor bolt on's I wouldn't bother. Also I remember reading a test a couple years back where they did a dyno comparison on a 90's 240sx. They upgraded the exhuast system on the car, one was a crushed bent system and one was a mandrel bent system. The difference on the dyno was 1hp which most consider neglegable gains since there are many other factors that can contribute to a 1-3hp gain on a dyno.
It makes a difference when your making power, if you have a upgraded turbo and you boost the car more than 14psi regularly I would suggest a true mandrel 3" system. For anything over 300hp for that matter. I get a deal on those systems if your interested with Magnaflow mufflers.