Brake Rotors - Which rotors are right for me?
Just like choosing brake pads, the answer to that question depends on the type of driving you do.
BMW brake rotors are excellent - vented for cooling and oversized for the application. When combined with a capable brake pad they provide superior braking performance even under mild track and autocross situations. The venting and sizing is key because brake rotors store heat (see below) and the added mass of oversized discs helps prevent "warping". In almost twenty years of BMW racing and tuning I have never seen a warped BMW rotor.
Brake Rotor Brands:
Standard OEM Direct Replacement
The exceptions being Zimmermann and Pagid rotors.
Zimmerman Coated OEM Direct Replacement
(still acceptable to use on the street but the slotting is just for looks)
Cross-Drilled and Slotted Street
M Sport, M Performance, or Euro (selected BMWs only)
Aftermarket 2-Piece Floating
Cast Iron vs. Carbon Rotors
F1 cars and Le Mans Prototype cars use carbon brakes - they must be superior right?
Yes, but only at the top levels of automotive technology. For racecars it makes the most sense: lighter, better heat dissipation, and slower wear rate than cast iron. With high speeds on high downforce the braking force is short and violent. Carbon ceramic brakes (CCB) mean the drivers can brake later and shed speed quicker. And they don't get the brake fade that cast iron rotors and ferrous pads can have. The same advantages benefit today's ultra-fast supercars. With horsepower at or near four figures, the need for seriously impressive brakes has become a requirement. CCB simply perform better from the silly speeds these cars are capable of. To achieve the same results from iron brakes would require pads that are too aggressive for the street and humongous rotor sizes (with accompanying weight). Those pads would be ineffective at slow speeds, make noise, with large amounts of dust. CCB brakes came hand-in-hand with more power. It also makes for great marketing.
For today's regular street and track cars the CCB simply don't make financial sense. Prices have improved since carbon brakes first appeared as OEM equipment in 2002. Still, a set of Brembo carbon brakes run in the tens of thousands, and BMW's CCB brakes option are over $10k. For the rest of us the only real advantage would be the longevity but it will be a long time for them to be financially sensible. Remember, drum brakes were still common well into the 1960s even after discs and calipers stunned the field at Le Mans in 1953.
Even though this article is several years old it's still a great read on the impact of CCB: http://www.autoguide.com/auto-news/2012/10/why-you-should-or-shouldnt-up...
More Info (in very simplified terms...)
It's a common misconception that simply changing brake rotors will improve your stopping performance. This will only prove true under extreme conditions - track events and racing - and only with rotors such as BMW's Euro and Performance parts. For the majority of us, the standard OEM replacement rotors will be more than sufficient for the type of driving that we do.
The brake rotor acts as a heat bank. Heat is stored in the cast iron of the rotor as the brake pads create friction. The central vanes of the rotor direct air through the casting, venting the rotor of the heat and also cooling it at the same time. A larger rotor has more capacity to store heat and can thus, provide better stopping performance for greater lengths of time. This is one reason why racecars typically have big brakes. Too small of a rotor for the application will lead to the heat being trapped with the rotor and overheating the rest of the brake system. Extreme heat can warp a rotor but braking is not sustained friction - it's only a matter of seconds that you're on the brakes. You will have plenty of warning of excessive heat before you ever warp a rotor: the brake fluid will boil and the pads will fade.
This makes a difference in your rotor choice because a standard brake rotor has more mass and will act as a better heat bank than one that has been drilled and/or slotted. It has become necessary to drill or slot the rotors for other reasons - mainly to allow an exit for gases that build between the pad and the rotor. The gas needs an escape so that it does not block the application of the pad to the rotor. But newer technology has made these types of rotors nearly obsolete (although they look pretty cool on a street car). None of the street pads produce this gas and there are very few race pads that still do. One benefit to slotted or drilled rotors is that the edges of the holes and slots can graze the pad material, which will decrease pad life but help maintain heat in the pad (critical for track pads). One other thing to note: a drilled & slotted rotor will have less mass and less braking surface than any rotor - these are mainly for looks.
What does all of this mean to you?
On a street-driven car with street pads, the typical driver will see no benefit to using a cross-drilled, slotted, or two-piece Motorsport rotor over a standard OEM replacement rotor. There are small benefits to using one of these on a street car - looks/aesthetics, drainage for water, for instance. There's nothing wrong with using a different rotor on the street and we would not discourage that. But if you're looking for better braking on the street, first upgrade your pads and consider the rotor design to be an aesthetic choice.
For more in-depth and scientific explanations, see StopTech's excellent technical pages: