I was briefly (1.5 years) A-list. Here are a couple things you should be aware of:
1. A lot of AA employees are unhappy-- just in a general way. There are plenty of great employees for sure, but look around in this forum for some of the threads at the treatment people get. It's completely hit or miss and you have to hope when you're having a travel issue that you get one of the great ones and not one with an IDGAF attitude. I was always impressed by how much everyone who worked at WN seemed genuinely nice. Clearly WN does something differently with their hiring. So that will be something big to get used to.
2. As far as MCE, look at the Oasis thread. If you're used to WN's pitch, especially if you are used to getting the exit row, you'll find Oasis to be really tight, even in MCE. While that's not on every plane, it's the future of the fleet.
3. A bright spot is the regional fleet. There have been lots of E175s added, with first class (often an easier upgrade than mainline) and 2x2 seating in main cabin. Those I would say are clearly superior to traveling on WN.
4. AA is affected by the MAX as well, though not to the extent of WN.
My personal opinion is if you are flying enough that things like schedule reliability and IRROPS handling are as important to you as the occasional upgrade, you'll probably be happier at DL. That being said, as a lower level elite (AA Plat) you'll have an easier time getting upgrades generally than on DL because of the 500-mile upgrade ("sticker") system, which means you're not competing with the entire pool of elites for every upgrade (just those who decided to request it).