So what do you think is the better disc set.
Hayes HFX Mag or XTR with XT levers.

Sorry but IMO its the Formula B4SL super light super trick!

When you say XC, I assume you mean low weight, medium power stoppers.

If this is the case I think the options are Formula B4 (Pro or SL dependent on what you want to spend), Hope mini or Magura Marta (SL if you must have the carbon lever!).

The Hayes and the XT certainly have plenty of power, but how much of it will you need on an average XC? They're also both quite a bit heavier.

If you want reasonably light (but not ultra-light) I'd go for the minis. You can get them for under £100.

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.....but it still has to work!