Tough call, because rush hour traffic is so unpredictable. Yes it can certainly be crawling some days as you'd expect it to in which case taking the CTA would make sense over a cab. But I have seen days where traffic is moving reasonably fast even during rush hour. If you can get an current traffic report when you arrive, that would be ideal.

One possibility is to take the EL to the Chicago Ave stop. Get off there and hail a cab and have him take you to McCormick. Fastest way is probably just to go East on Chicago all the way to the end where you get to the Lake Shore Drive entrance, then Lake Shore to McCormick. With Taste going on right now, it's probably the best way to avoid the traffic glut in the surrounding areas.