What's the overall goal of the OP? If it's to go to a major don't worry about type just go with who gets you to left seat the quickest then who can offer instructor /check airman advancement. Going to an ACMI outfit with 737 will ensure that you be flying 737 but that's about it. Slow time building, few advancement opportunities, lower pay, less bennies but the crews you work with will be the most rememberable you'll ever work with.
Just based on flying the 737 and nothing else I'd go to sun country. Either they'll continue to plug away and be in business for a while or get bought by someone . I'd consider them a step up from swift or southern