Traffic on the I-90 can be bad, but if you arrive at ORD in the afternoon most of the traffic will be in the opposite direction, with people driving back to the north suburbs after work. It shouldn't take you more than 50-60 mins to get downtown by car.

The El is OK and cheap, and the cars have luggage spaces. Quite frankly it's not such an exciting ride until you get to Division though. Also, both trains and some station platforms in the Loop can get very crowded in late afternoon, which can be a pain if you're carrying a lot of luggage.

Where are you staying? You should consider how far the nearest CTA stop is from your final destination.