100% of your AA itineraries involve some RJs, while many Delta ones won't (LIT). Not sure how you feel about that, but I'd think carefully about it in terms of comfort and transfer experiences.

I'd also think hard about which carrier you naturally tend to fly more. Think about the destinations you most often go to, and which carrier is most convenient to those. You don't want to be struggling for top tier on a carrier that involves taking less convenient flights on a regular basis.

You mentioned that you'd be top tier in one and bottom or mid in the other. Mid tier on AA is pretty great, with double miles and lounge access on flights touching places outside of North America. Bottom tier (gold) really isn't much in my opinion. With top tier you get unusually good service from phone reps, free north america upgrades, and the 8 international upgrades mentioned by others.

This is only heresay, but AA has a relatively good reputation for award redemption, especially versus Delta.