Slotted rotors offer improved bite (initial onset of braking) and slightly higher friction level than smooth rotors.  Slots prevent reduction in friction due to pad outgassing or brake dust trapped between pads and rotors.  Pad coefficient of friction is maintained over the lifetime of the pads, since slots shave away glaze formation and expose fresh pad surface each time brakes are applied.  Slotted rotors are by far the number one choice for cars used in competition or open track events.  Best choice for heavy trucks and SUVs, particularly when extra bite is needed for towing.  Disadvantages include slightly reduced pad life, some low frequency rumble and pedal flutter when braking hard from high speeds.  If the slots are improperly machined all the way to the outside edges, then rotors may develop cracks sooner than plain or properly slotted rotors. 

Drilled rotors offer slightly more bite and friction than slotted rotors.  As with slotted rotors, pad coefficient of friction remains consistent over their lifetime.  Wet bite is improved over plain and slotted rotors, so these may be the best choice for areas with heavy rainfall, like Seattle or Singapore.  Weight is reduced by about 0.2 pounds per rotor, depending on size and drill pattern.  Disadvantages include possible uneven rotor wear, typically concentric groove formation, although this is mostly an aesthetic concern.  A major disadvantage is accelerated formation and spreading of cracks under racing conditions.  For this reason, drilled rotors should be avoided for track cars, unless required by the rules.  A common piece of misinformation is that they have lower performance than smooth rotors, due to reduced surface area and are for looks only.  This is false.

Slotted & Drilled rotors offer a compromise, midway between the benefits of slotted rotors and drilled rotors.  These are fine for street applications, but should be avoided for track cars.  For the first time, slotted & drilled rotors are starting to appear on some cars as delivered from the factory, including models from BMW and Mercedes.

2-Piece Floating rotors consist of an iron "friction ring" assembled with float hardware to an aluminum hat or mounting bell.  These are available slotted or drilled.  2-Piece rotors offer substantial weight savings over the 1-piece rotors described above, as much as 10 pounds per rotor, depending on the specific application.  The design allows the outer friction ring to expand as it's heated, without being constrained by the center section.  This prevents rotor "coning" and subsequent tapered pad wear and spongy pedal.  Because the center section and friction ring are fabricated from dissimilar materials, conductive heat transfer is reduced, lowering wheel bearing temperatures dramatically.  Outer friction rings may be replaced when worn, while reusing the center hat, at substantial cost savings.  Click HERE for more details on the hardware and assembly of StopTech floating rotors.  For track cars, these are absolute best choice and should be used when available.  A disadvantage is significantly higher initial cost. For street cars driven in high-salt/chemical environments, corrosion between the iron friction rings, aluminum hats, and float hardware may lead to reduced product lifetime.  This can be mitigated by flushing the brakes with water frequently or swapping back and forth between "summer" and "winter" brakes.