Pro ORD:
- good connection into twon in case of long layovers
- great immigration experience when arriving from FRA on 945 (never a line at all)
- overall better airport experience
- somewhat better RCC
- better Airport Hotel options in case something goes really wrong...

Con ORD
- need to change terminals on Intl. arrival
- weather related problems

Pro IAD
- easy immigration & security at midfield terminal if on a off peak flight (eg. 951)
- Great LH Lounge in case of longer layovers

Con IAD
- incredibly long lines at immigration during peak times
- lousy RCCs
- crowded, filthy overall impression
- poor connecion to downtown