The airline is a good airline and the HHH Terminal is a nice alternative to the Lindbergh. However, IMO, the program is not worth it. It's not a loyalty program, so there's no real benefit for you to stay with them. The only thing you get is a leather seat and a hot meal for a 3 hour flight and an extra inch or two in FC.

So, to accurately compare, I would think you have to look at apples and apples. How much will the Sun Country fare cost you (with the VIP program fees built in) vs. the NW fares, considering that NW tends to match Sun Country flight-for-flight on fares (at least ex-MSP). After you have the costs for each, then you can decide whether your preboarding, elite checkpoint, WorldClub access, bonus FF miles (heck FF miles at all since SY doesn't do that), and schedules are worth switching over.

IMHO, it's not, even with the draconian cuts/"enhancements" from NW lately. I still value my elite status and my WP miles are still worth something for now.

It would also be fair to calculate what it would cost you to make some MRs to get you up to NW Plat so you'd have a better shot at upgrades and compare that.

$.02, since you asked.