I decided to compare Beedrill and Pheromosa side by side, and here are my results.
Beedrill has more raw power, due to its higher attack stat and Adaptability. It also has marginally more physical bulk and almost twice the special bulk of Pheromosa. However, while Beedrill is very fast, Pheromosa is faster, can hold an item, is not a mega, and has much higher special attack.
0 Atk Mew Giga Impact vs. 0 HP / 0 Def Beedrill: 219-258 (80.8 - 95.2%)
0 Atk Mew Giga Impact vs. 0 HP / 0 Def Pheromosa: 231-272 (81.6 - 96.1%)
0 SpA Mew Hyper Beam vs. 0 HP / 0 SpD Beedrill: 130-153 (47.9 - 56.4%)
0 SpA Mew Hyper Beam vs. 0 HP / 0 SpD Pheromosa: 231-272 (81.6 - 96.1%)
Both Beedrill's and Pheromosa's main Bug attack is U-Turn, but they both have a few others. Beedrill has X-Scissor for extra Bug power, Pin Missile to break potential Sashes, and Fell Stinger to give a HUGE power boost after killing something. Pheromosa has Bug Buzz, which is a special attack, and Lunge, which can weaken physical attackers.
Beedrill's only option for a Poison attack is Poison Jab, while Pheromosa has the option of High Jump Kick and Focus Blast for power, and Double Kick if you really hate Mamoswine and Smeargle (shoutout to AmShowdownNoob).
Now for coverage outside of STABs. Both Beedrill and Pheromosa have access to attacks of the other's type, with Beedrill having access to Brick Break, and Pheromosa having access to Poison Jab. Beedrill also has Dark coverage in Knock Off and Pursuit (both of which provide additional utility), Ground coverage in Drill Run, and pseudo-coverage in Endeavor. Pheromosa has Ice coverage with Ice Beam or Blizzard, as well as Hidden Power for weak but customizable coverage.
Both Beedrill and Pheromosa have limited access to boosting moves. Beedrill gets Swords Dance, Pheromosa gets Quiver Dance, and both get Agility. Swords Dance provides a larger boost in power, and Beedrill's superior bulk may let it set up more easily. However, Quiver Dance's boost to Special Defense, as well as Pheromosa's ability to hold a Sash, means that Pheromosa might actually have an easier time setting up.
As for utility options, Beedrill has access to the aforementioned Knock Off and Pursuit, as well as Toxic Spikes, Defog, and Tailwind. Pheromosa, on the other hand, has Rapid Spin and Taunt.
The different typings of Beedrill and Pheromosa lend their own defensive advantages and disadvantages. Both Beedrill and Pheromosa are weak to Fire, Flying, and Psychic, and resistant to Fighting, Bug, and Grass. Beedrill's Poison type makes it resistant to Fairy and Poison, as well as increasing its resistance to Fighting and Grass, but it nullifies Bug's resistance to Ground. Pheromosa's Fighting type make it resistant to Dark and nullifies Bug's weakness to Rock, but makes it weak to Fairy and worsens its Flying weakness.
Those are all the points I could come up with. I hope this makes discussion a bit easier.