Unfortunately, Alakazam has lost favor in competitive battling over various other Pokémon that are used in the roles he used to fill in previous generations, with Azelf, Colcartona, and Porygon-Z all overtaking him as general and set-up sweepers, and Chandelure taking over as a special revenge killer. Alakazam's main issue in later generations is his low endurance, which becomes a big problem with more and more hard-hitting foes that can take an unboosted-hit and KO Zam before he gets a chance to set up Calm Mind, strike a second time, or even use Recover introduced in each generation, diminishing his usefulness over slower, yet relatively more durable sweepers which can stand up to such attacks.
While Gardevoir is obviously not as efficient a special sweeper as Alakazam (or any of the other Pokémon listed that Alakazam trails behind), her special ability in trace makes her a highly specialized counter for popular strategies such as Swift Swim and the ever dreadful Inconsistent that would normally mow through an entire team. Her typing, offenses, and speed allow Gardevoir to differentiate herself over Porygon2 (the only other fully-evolved tracer in the game) as the more "offensive" of the two choices, giving her a niche that no other Pokémon could easily fulfill.
As for my own choice, while I have been an avid Alakazam user for the first three generations, Gardevoir has to take my vote, mostly because of her utility.
