I have been reading the details on these two. It seems the BOE might have more friction on the rotor vs the ULTRAdisc. It also seems that the BOE rotor might last a shorter amount of time. However the BOE is a lower cost and shows 10lbs lighter weight than the ULTRAdisc.
The BOE HP Fixed Rotors look very similar to the ULTRAdisc. However the ULTRAdisc claims "True floating rotor = With No rattles or Uneven Wear"
Where BOE has a totally different looking rotor for full floating.
Advise / input from actual use? Is BOE HP Fixed really 10lb lighter?
Also, found some posts about short hat life for the BOE. Is this still true?
This would be for track / Autocross use.
Here is just some general information about disc brakes. This information is not specific to either of the brands you had mentioned.
The friction coefficient has more to do with the brake pads used rather than the specific metal used in the friction ring of the disc brake. Another observation is that drilled discs tend to act like a cheese grater on the brake pads. This can be seen on well used discs - worn groves in the pads and disc coincident with the drilled holes. See photo below. Drilled discs also crack under heavy/track use. Keep away from drilled discs for the aforementioned reasons.
There are many ways to "float" a friction ring on a disc brake. Some good, others not so much. The primary reason for floating the friction rings is to eliminate the very small misalignment that you get as a result of production tolerances. The other reason is to eliminate the thermal stresses between the bell (hat) and the friction ring. (Will be explained later.) As for the tolerance stack up, there is no way to get the friction ring to run absolutely true to the wheel hub. The tolerance stack up just makes it economically impossible. The misalignment (wobble) in the friction ring, knocks the brake pads back into the calipers and creates a very small gap between the friction ring and the brake pad. That adds brake pedal travel and you loose braking feel. If you don't care about brake feel, you can save yourself a bunch of money by going with fixed discs.
But, if you do want the ultimate in brake performance, you really need to go with full floating discs. As mentioned earlier, the bell (hat) of a fixed disc needs to be heavier/stronger to take the thermal stresses along with the braking loads versus a full floating disc. The sad news is that I don't know of a single aftermarket disc brake that has dedicated the engineering resources necessary to design a proper full floating disc. They just "float" the heavier fixed disc bell (hat) design and call it a day. It's just in the economics of it all.
There are many acceptable ways of "floating" a friction ring. The best way is to design float between the friction ring mounting points and the fasteners because the fasteners are iron based (steel) and the friction rings are iron based too. They both are of similar hardness, compared to aluminium. The other advantage is that the fasteners need to be replaced when the friction rings are replaced...so no additional cost in use. The bells (hats) can be reused much longer. The problem is that that type of friction ring is bespoke to each design and significantly more expensive than fixed disc friction rings. Your choices are very limited in friction rings, so your chances of finding the right friction ring are very slim and significantly more expensive than a fixed disc friction ring. Therefore, you are pretty much limited to floating a fixed disc friction ring design because of choices/selection. Since the "float" will be between the aluminium bell (hat) and the fasteners/bobbins, the design must consider the relative hardness between the aluminium bell (hat) and the fastener/bobbin system. Because the fasteners are fixed to the friction ring and allowed to float in the aluminium bell (hat), there will be relative movement between the aluminum and the faster/bobbin system. Since the aluminium is softer, the aluminum will wear before the steel fasteners/bobbins will. If this has not been considered in the design, the floating disc will become looser than acceptable and begin to make noise...rattle. That is why bobbins are used between the aluminum bell (hat) and fixed disc fastener. The bobbins are designed to distribute the load into the aluminium. Again, just because there are bobbins, does not mean that they were optimized to minimize the ware in the aluminium bell (hat).
The other design consideration for friction ring is the annular friction surface. Even Lotus got that wrong with their 308mm BBK. The depth of the annular surface needs to match the depth of the brake pad. Here you can see 2 of the problems I've described in the photo below. The inner edge of the annular surface is too wide for the brake pad and you can see the ridges in the annular surface of the friction ring caused by the drilled holes.
When the annular surface is too deep, the disc is heavier than it needs to be and because the brake pad only heats up the swept area of the annular surface and insulates only the swept area, that will cause internal thermal stresses. These internal thermal stresses within the friction ring can cause coning of the friction ring. Neither condition is desired in a high performance disc brake system. The reason Lotus chose the 308mm BBK disc is that they already had it lying on the shelf for the (GM) Vauxhall VX220. But the VX220 uses a 52mm (I think) brake pad depth. That leaves 6mm of the inner surface of the annular swept area unused. The depth of the Lotus brake pads are 46mm.