Take the early class date. Trust me, one class date can mean years of seniority disadvantage if you end up in a small base or bidding pool. This is not hypothetical, I'm living it and have been for years.

Your have two goals in this:

1) Minimize employment air-gap to limit exposure to total unemployment in the event of a sudden industry catastrophe. Very small chance but it could mean the difference between having a job, furloughed but coming back some day with a seniority number, or starting all over.

2) Keeping eligibility for re-hire at the ACMI.

These two considerations are at odds with each other. If you resign the day before your AS class date, then you will eliminate the air gap but burn the rehire bridge.

Generally the way to ensure rehire eligibility is two give two weeks notice. If you do that you might very well get a two-week air gap for your trouble.

If AS is where you plan to stay, then maybe the rehire eligibility doesn't matter although there is a danger that AS will get wind of your inconsiderate departure and might even un-hire you.

All told I think you're better off risking the two-week air gap (but I wouldn't give more notice than that unless the ACMI has some weird policy). Maybe they'll keep you in the training program as others have mentioned.

Keep in mind there is some risk taking a checkride. If you bust for some reason, then your AS job could be in jeopardy. While a 74 type would be really cool, I might avoid the checkride if possible since a legacy airline would be even cooler. But you're in the best position to judge the risk there, considering the training environment, your aptitude and your partner's aptitude.

I would not worry at all about "incomplete training". The explanation is "I left for a job at a legacy airline". Worst case if you try for big three later you might have to go to a job fair and explain in person why you checked that box.