It's fairly well documented that the Mk. 2 weapons offer an extra mod slot over the standard version of the weapon. That extra mod slot is always unlocked, so at the beginning of play you've got two open mod slots instead of just one.

Not so well documented is that the Mk. 2 weapons level-up faster than the standard versions. Here's how much XP you need to get the first, second, and third (mastery) levels (K means 1000, of course):
  • Regular weapons: 14K - 46K - 78K
  • Mk. 2 weapons: 10K - 20K - 30K


Not documented at all, as far as I can tell, is that for Switch users, the Arwing levels up faster than the other ships do once you get beyond the second level:
  • Most ships: 8K - 26K - 82K - 118K - 164K
  • Arwing: 8K - 26K - 54K - 92K - 140K

I haven't tried all of the ships enough to get past the second level, so it's possible there may be ships other than Arwing that level up faster. Feel free to add a comment with your own experience.