I have been a low- to mid-tier elite on both airlines. Honestly, the flight experience is pretty similar. If upgrades are not super important to you (as you stated in your last post) then it will be even more similar (otherwise, route/day/time makes a big difference in upg%). My advice is choose based on which airline has a schedule that makes your life easier. Since you are going to be flying a lot, an hour saved here or there makes a big difference.
When I started flying routes where AA had the most convenient schedule, that's when I switched to AA. You still haven't told us which city pairs you'll be flying often, but you did hint that you have to adjust timing of your flight in order to stay on Delta and avoid AA-- that might be a point in AA's favor. If both AA and DL offer similar schedules and connections for you (and that's a BIG if), I think DL has a slight edge in terms of operations. Meaning, everything else being the same I would choose a DL operated flight over an AA operated flight However, it's a small difference and I wouldn't make too much of it.