Both have their advantages.
Columbia's Black Messenger, one of my favorite balls, is a urethane called Flexathane, and no I don't know exactly what the difference is. The core on this ball, as on the Stingray and the Buzzsaw XXL, causes the ball to flare quite a bit. This can do good things to the ball, not the least of which is more backend reaction.
The old Hammers, some of the best urethanes ever made, are lower-flaring balls. So they more or less rolled how you released them without too much axis migration. This made them fairly consistent (read how many people on this board want their Black and Red Hammers back). For someone like me, who has a lot of forward-rotation to their released, this causes a phenomenon that is referred to as a "heavy roll": a roll which seems to drive through the pins especially well dispite slow ball speed, not flush in the pocket, etc. My low-flaring Piranha does this as well.
Some of the remakes, however, are traditional three piece construction urethanes. These tend to be somewhat cover heavy, don't turn in oil quite as well and tend to not flip on the backend as hard, and don't hit as hard. With them having more weight distributed through the ball as opposed to concentrated at the core, it's harder to get revolutions on the ball (less leverage to the weight). There are bowlers that grew up with this stuff though and love to throw them (my dad loves his Black Scout, and a guy in our league averaged 210 with a Claw... big-handed guy.)
If any one has more to had, or corrections to make, chime in. This is the way I understand this stuff and if there's stuff missing let us know.
Nate
