Well, probably too late now, but I think I can help answer your questions.
Source: I have gone to SF, Seattle and Vancouver congresses, every year since about 3 years ago. Haven't taken classes at any of these except the first year.
I am a lead that currently lives in seattle and go to SF 2x a month and Vancouver 1x a month.
A couple of 'facts'
1. SF congress is way bigger. Lots more dancers; more "stars"; more high quality dancers. it also ended up being more expensive.
2. SF congress...the performances...HOURS. OF MY LIFE. I WANT BACK. the performances themselves were good; just way too many to sit through; got bored, even for great performances. Just wanted out. Went back to hotel and slept.
3. SF congress: Sunday night. Stay for sunday night. Best dancing. I felt inspired...haven't felt that in a while you know? was great last year.
4. SF congress and seattle I think share costs a lot of the performers will be the same; the congresses are practically back to back, so the performers just come up to Seattle, hang for a bit, and do the second congress. So you won't get any difference in performances, "stars to dance with", etc.
5. Seattle congress is about..I dunno...1/3? the size of SF on the busiest night saturday. But, on the other hand, being small means: classes you can see the instructors; easier chance of dancing with "stars"; less ******** (transportation, costs, etc.)
6. Seattle congress is RIGHT by the airport; SF congress is in oakland, which is far from SFO but not even that cose to OAK airport.
7. Both SF and Seattle are lead heavy; if you are follow, good times ahead.
A couple of 'opinions'
1. Seattle bachata and kizomba is better then SF and Vancouver, and not just by a "bit"; by a "lot". But it's not that the "best of the best" are a lot better; it's just that the distribution is moved along and squished, so the worst is better, the average is better, etc.
2. On1 versus on 2.
"Top" dancers in SF and Vncouver can dance both on 1 and on2.
Seattle has more "top" dancers that will only stick to one or the other.
I'm excluding some of the bitchy on2 people from all 3 cities

3. "Fun-ness". I have had the most fun in:
* vancouver. Way, way, way more follows then leads - I'm a lead so that was nice. Very highly organized; no ********, **** stars and ends on time; every dresses way the **** up.
* seattle. My homies, my town; small, intimate; easy to pick out all the best dancers and easy to get opps to dance with "stars". No one really dresses up that much.
* SF. lots of great dancers. Sunday especially amazing. but bachata not so good; everything runs late. Much bigger congress.