My tuppence.
I would have avoided Avid, but my last two pairs of Elixir’s and DB5’s have been faultless and cheap. Outright power is on a par with Shimano, but they’re more progressive rather than the more on/off feel of the Shimano servo wave brakes. They also seem to have dropped the Avid name from all but the cheapest models which I’d take as an admission that things weren’t right and they’ve improved if they’re prepared to re-brand them under SRAM.
I’ve also got Shimano XT’s, as I said, different, but no more powerful when used hard. The downside of Shimano is the rotors, there have been several cases of people melting Ice tech rotors, apparently they work well for cooling your brakes, but only upto a point then they fail catastrophically! They also have an incredibly thin steel surface, so I’d hesitate using them in winter too unless I was minted. Having said all that, you could use the standard rotors and be no worse off than using Avid/SRAM.