If you're background is clean and you're big-six material in all respects, if you stay at SKW, take the first upgrade, the bigs will almost certainly call in the next few years.
In my mind, the key question with sun country is how long to upgrade? And how much will you fly as a junior CA.
If you're going to the big-six for sure, then take the fastest route.
If you have any background glitches, or are risk-averse, then maybe better to lock in the decent job at sun country now, and maybe move on later, even if it delays your ultimate career progression.
But I would most definitely ask someone at sun country how their training, schedules, etc are (I have no idea).