Ok................
First, you can download the DL schedule, which includes plane information, from the DL website. Also, many travel sites (itn.com, etour, etc) will indicate the plane type for you.
To your specific questions:
I'd probably lean towards MDW as they will probably be cheaper than ORD (Southwest influence). Secondly, MDW uses 737-800 equipment as opposed to 737-200 and MD88's. Now, there might be a good chance that the equipment out of MDW might be re-positioned shuttle aircraft. Not a bad thing though, no FC, but a much better coach product than DL mainline.
Don't expect much from either chicago airport to ATL. ATL to LAX will be some sort of snack or meal and a movie.
Also, check out
www.seatguru.com for suggestions on good seat selections.
Good luck.