You say you have just under a year at SW, so let's call it a year for math's sake. If you have a CJO (not sure if this was hypothetically or you actually have one), once you accept and actually get a class date could be several months down the road. If you don't have a CJO, then you have to factor in the time of applying, getting interviewed, getting the CJO, etc.
I'm not sure how backed up Delta was with classes, but last I heard from some buddies, it was 6+ months (my gouge could be old). If this is still true, once you're on property, you're talking about a minimum of ~18 months from your original SWA hire date. So any benefit you might get from an early upgrade could be a wash.
Guys like to point out your seniority when you retire, but I honestly don't care what number I'll be on my last day. What will my last 3-5 years look like? I can't speak for everyone, but at 38 years old now, I value QoL over pay, so I can only imagine that gap will get even larger the older I get. I don't really care if I can barely hold WB CA at 63 yrs old because my QoL will probably suck. What company will give me the greatest flexibility to add/drop/trade etc that will allow me to live the life I want to live? That's the company I'd go with. Seniority obviously plays a huge role there, but there are scheduling and work rule differences that come into play too.
I'd stick it out if I were you but it's not a bad problem to have. Good luck!