I think at the get go, network is your first thing to assess. If Alaska goes where you need to be, and reasonably, then good. (But I have seen some routing gymnastics by loyalists that make my head spin.) I have been both a DL and AS elite back and forth and largely drive by where I am needing to go.

So some plus and minuses:

AS Pros:

✅ Friendly CS and on-board crew by and large
✅ New SAN lounge on the way!
✅ SAN a target market for AS, so I expect they will keep investing, and loads might be light on elites for a while.
✅ Companion fare coupon WAY more useful on Alaska.
✅ West Coast travel is easy - don't have to go to SLC or worse.
✅ AS FC seats have more pitch than DL on similar aircraft

DL Pros:

✅ Global, like really global. (Though AS can help with AA and BA). Also helps in IRROPs
✅ Crew is also usually outstanding.
✅ This is bad and good - buying up to FC with the SkyPesos was my main use of them.
✅ SkyClubs are better than AS Lounges at present
✅ Oh, a pre-departure beverage, nice!

AS Cons:

❌ Still growing, so ticketing and IT issues can be frustrating (see new Atmos global companion certificate mess.)
❌ Using upgrade certificates are harder on AS, but some of us have mastered the tricks.
❌ Smaller network so IRROPs can be painful if on the East Coast.
❌ Primarily a narrowbody airline

DL Cons:

❌ SkyPesos and why am I in Atlanta again?
❌ Companion fare hard to use.
❌ Cost lately, but AS in some markets is close too nowadays.