Welcome to FT. Biggest differences between AC and DL:
DL flights generally depart and arrive on time - quite unlike AC and its consistently brutal OTP.
DL treats you like you're their best customer ever - even if you have no / low status.
Upgrades flying ex-YYZ are generally not that difficult once you attain status.
DL Skylounges are generally pretty good, unlike many of the overcrowded Maple Leaf lounges - some of which are little more than glorified refugee camps.
If you fly primarily to the US, you'll be happier on DL - especially with so many of AC's transborder routes having been roughed rouged.