You have status on United so you would probably want to keep it. ORD has a lot of competition between UA and AA and I don't know what your travel policy is. If you think you are going to fly AA a lot more now then the Plat Challenge is the way to go. Wherever upgrades will be easiest for you to get then that would be the way to go.

I, too, am a Hilton man. I have been with Hilton for a long time and also use Starwood as my backup. I like the 20K points for 25K miles conversion with SPG. I travel a lot off of the beaten path and as a necessity have a lot of stays with PC (love those HIE cinnamon rolls!). I guess it all comes down to personal preference. I like the motto, 'If it's not broke, don't fix it.' So if Hilton has been good to you then keep double-dipping. It makes up for some of the shortcomings when compared to other programs.