Ok, I've chosen SRAM XO (at the mo) which rear mech? Medium or Long Cage?
Will be running triple ring up front and 9spd cassette. Bike is a SC Chameleon.

Also SRAM cassette or Shimano?

Cheers

Long cage is absolutely guaranteed to work in every single gear combination without problem.

There's a very good chance than a medium cage will also be fine, but there's a slight chance it will either get ripped in half when you go big/big, or the chain will be very slack when you go small/small.

There will now be 100 people along to say 'I run that combination and medium is fine'.

you can run a medium cage with 3×9 but it is recommended to run long cage.

I prefer medium cages if for no other reason than it looks tidier and I think it slaps less but you have the issue that you need to shorten the chain so much with a med cage that if you cross chain (big to big ring) by mistake you can end up in trouble. ofc people will tell you "be careful you can get in trouble with cross chaining" which just allows you to look all imperious and say "well, so long as I don't ride like a noob who doesn't know what gear he's in then I'll be OK, no? how is that new long cage mech treating you btw?" … but it's up to you.

as for cassettes, I buy shimano ones, but for no good reason that I can give you

I run medium cage on 3×9, you shouldn't be going big to big anyway.

If if you do accidentally go big to big, rather than ripping the mech apart it just grumbles a bit and you're able to sort it.

Yeah medium cage is fine…got a 34 tooth rear sprocket as well…unlike Freegan BF I use Sram cassettes cos I like the shiny red carrier…is that a good reason?

Re XO vs XTR…most of the really blinged up mega bikes are on XO!

I use Sram cassettes cos I like the shiny red carrier…is that a good reason?

perfectly good 😀

+1 for above!! I run Sram Cassettes with the gold bits, and have gold jockey wheels in the mech (X0 mech), i have always wondered whether this was a little too far?? Looks good though!!

i use a long for 3 x9 and medium for 2×9