6-bolt vs centre-lock: pros and cons? I am having a new set of wheels built, and am trying to decide on whether to go with 6-bolt or centre-lock rotor interface.

What are the pros and cons of the two systems?
Centerlock largely exists because since the beginning of modern disc brakes for bikes, Shimano has more or less alone had a weird cautionary take on the physics of rotor bolts theoretically being able to loosen in the six-bolt design. That's why their six-bolt rotors have always come with various retention systems for the bolts that nobody else bothers with.

Centerlock doesn't accomplish anything special and doesn't allow for lighter hubs or rotors (at least in a meaningful sense). Shimano rotors are excellent so it's not like you're trapped if you have centerlock hubs, but you do have fewer options, and centerlock rotors are inherently more expensive at the low end because they can't just be flat pieces of steel.
Centerlock has the disadvantage that carrying a lockring tool plus something to turn it isn't practical, so the best you can do if you severely bend a rotor on a ride is undo the caliper mounting bolts and try to hang the caliper out of the way, which is awkward. Even though the risk here is small and theoretical, it's enough of a potential day ruiner that given centerlock has little to no functional advantage, there's an argument to avoid it if all else is equal. The catch is that all else is often not equal, because Shimano hubs are some of the best value choices.
Six-bolt does have the disadvantage that bolts getting seized, heads getting stripped, and/or hub threads getting damaged are all things that happen sometimes, whereas centerlock tends to be free from any such problems. Having one big high-torque fastener does create a more reliable interface in that sense. But it's a pretty fringe concern; six-bolt is perfectly robust and very few people will ever have to deal with a seized rotor bolt. However, in all fairness it can be a difficult enough problem to deal with when it does happen that centerlock usually looks pretty good by the second hour in.