Jr. captain at Delta is 1.5 years, hiring in the neighborhood of 1000 per year. Lots of jets on order, with some uncertainty regarding the end state growth - but my money is on net gain. I'm not up to speed on the similar stats at WN, but I don't think they are anywhere near comparable.
I will fly any safely maintained transport category jet for the right $ and schedule. But let's face it, the 737 has the worst flight deck of any mainstream in-service airliner. If you value your hearing, ergonomics and actually enjoy a good view from your office, that dinosaur is last on the list of preferred work environments. Not trying to offend anybody, but it's a factor. Delta has them too, but other aircraft choices abound. Like others have said all around this forum, your question is moot unless you have offers from both or are working at one and the other calls. And you'd be silly not to apply to both and all the other viable players. Write us back when it's a problem!