Either boat would be an excellent choice. I've sailed on C22s and own a SJ21, and really, it's a toss-up. May come down to timing, condition, and price. There are about 50 other equally good candidates, but these two sound right and, as you say, are known quantities in plentiful supply.
The C22 is a heavier boat by about 700 lbs. It won't ghost along in 3 kts wind like the SJ21 (which is a light air killer
), but it will punch thru chop much better and offer more reassurance in bigger seas. Sails a bit dryer, too. C22 has much more
volume belowdecks, and something like privacy for the head. OTOH, that privacy involves basically stashing the head under your face in the V berth.
Keels: The C22 has a Big Dumb Slab of a keel, not particularly well-shaped; its keel pendant drags in the water, making humming sounds and corroding. Failure of this pendant is common, tho not a huge deal to fix. Keel does not retract all the way into the hull. The SJ21 has a fully-retracting keel, so it sits lower on its trailer. Its keel is foil-shaped, and the pendant stays out of the water. The downside is the winch is on the front of the compression post: hard to reach and it bangs you in the head while sleeping.
The C22 has a smaller cockpit than the SJ21. You can fit 3 adults and a kid it it, tho, so it's still functional. The SJ21 came in basically two versions, a doghouse coachroof (Mk1) and a flush deck (Mks2 and 3). The cockpit is roughly 14" longer on the Mk1, at the cost of interior volume. We can fit five adults (or four adults + two small kids) on our Mk1.
As for sailing qualities.... They are both fine, honestly.
I'd call the C22 more of a pocket keelboat, while the SJ21 is a dinghy on steroids. The San Juan is quicker to accelerate, surfs nicely on swells, and spins in its own length. It's substantially more tender than the C22, but it hardens up at 25 degrees heel and never feels scary. The C22 typically relies on a large genoa to supply drive, and I hear rumors it does not like to sail on main only. By contrast, the SJ21 has a small (100%, ~85 sqft) working jib and loves to sail on main or reefed main only. That's useful in bad weather or when shorthanded: you don't need to wrestle a big headsail, and you don't need to fuss with a jib when it's blowing hard or one parent is busy with the tyke.
Build quality is basically the same on both boats (middlin' fair) and cost of ownership will be roughly the same. Both set up quickly: budget 45 minutes on launch and 30 on takedown. Many C22 owners seem to rely on gin-pole mast raising systems, but that may be demographics more than anything. I can muscle up our SJ21 mast by myself, tho it's a grunt. If you can, find a marina or club that will let you store the boat on its trailer with the mast up. "Dry sailing" it will save buckets of time and you'll use the boat more often.
Cheers, and keep us appraised of your search!
ETA: Here's a C22 in Seattle area
(needs a keel cable, ahaaha) for $1k.