I want to build my first cx bike on a custom steel frame, so i have to choose which brakes would I want.
So here I am, asking you for help with decision, which brakes are better? I know, thet from WW point of view, cantis are better, cause they're lighter. But what about the performance?
Cantis are cheaper too, but the think I don't like is the silver braking surface on alu rims, and I can't afford for carbon wheelset for this moment, with discs there is no problem-there are a lot of 29 rims disc-only, full anodized. I know that's not good reason, but i like my bikes to look good.

I hope, someone will help me with decision

If it's custom, get the disc tabs on your frame so it's future proof. Use Avid Ultimate or Paul's cantis if you want to race right now, TRP mini v's if you don't race/ride in the mud and wait another year for the big boys to figure out discs. Actually wait two years for the big boy (Shimano) to make a Dura-Ace disc.

If you don't race and don't care at all about weight (you're on the wrong site ) get a disc only frame and use BB7s.

If you are into racing you should go for cantis brakes.
Disk for everything else (fun, commuting, training etc)

I think that weight is not that important in CX racing since there is no sustained climbing. I am using the avid bb7 sl brakes and I love them. You brake a lot on the cx course and I appreciate the extra power. I never liked the power of canti's. You can always upgrade and lighten up when hydros are perfected.

How close to the bar are your levers when you brake hard? I've read that some guys have the levers all the way to the bar. That alone would be a deal breaker for me.

limba, do you mean with BB7s? They can be a bit finnicky to adjust in order to avoid rub, and still not so wide open that you have enough lever pull before the bar gets in the way. Once they are set up though, they work beautifully. (This is my experience with Road BB7s)

Chances are that mountain BB7s are easier to set up with the longer cable pull. But you'd need to sort out the shifting with Di2 satellite switches or Retroshift CXVs.

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Bikes: Raceur, Beateur

Yes, BB7. I'm thinking about getting a Super X with Red and BB7 discs or going with Ultegra Di2 and Mini'V's.

As other said regarding racing. Cantis for sure. There were some stories about Nationals in other categories besides elite where the discs just didn't work at all and guys had to change to their pit bikes.

Ultegra Di2 is amazing for cross. Mine always shifted.... no issues with mud for the whole season. Plus, I never had to replace a shift cable due to contamination either.

You can also wear thicker gloves, if it's cold, and still be able to shift effectively.

Red is nice, because of the weight. But once you ride Di2, it's really hard to go back.

I ran mine with CX 8.4 mini V brakes as well. The new Shimano levers give you a lot of pad clearance.

Hmmm, which bike do you have? I think you have a 2012 Super X?
I crashed last year and broke my thumb. It hasn't healed properly and hurts to shift to the bigger chain ring. I'm thinking Di2 would solve that issue.
Why did you use CX8 and not 9's for your Ultegra shifters? Some say the 8's work better with either SRAM/Shimano shifter but TRP says you should use 9's with Shimano shifters.

While there is no sustained climbing, there is way more re-acceleration than in most other types of racing, plus the regular need to pick up and carry the bike. So bike weight is still an important factor in CX racing.

True and most everyone I know swaps their road training wheels with cross training or even race wheels if you use clincher. Going to disc takes that out of the equation.

Well said.

If you have some road wheels as well you can always swapped them back and forth. Disc well...

Eventually your road bike will have discs but that's another thread altogether.

Well there is also another way to look at it...
I swap wheels with my 29er XC bike.

Ever bike I have now is 700c, and the majority have discs.