Is it best to purchase advance tickets via the internet for the Chicago Art Institute and Museum of Science and Industry or wait until you are actually there?

Depends on when you are going. Midweek? Weekend?

Weekends after Labor Day are much busier than weekdays.

Also, check their calendars. Are there free days then? If so, they are also busier.

I would purchase via the internet...it's just one more long line that you will avoid. are you purchasing the citypass?

Wife and I will be in Chicago midweek (tue-thur) leave friday. No citypass since we do not plan on the Field or Shedd. Really looking forward to the Art Institute and Science Museum - along with the Navy Pier and of course the Loop walk with the Parks etc.
Science Museum won't tell you until you buy the ticket whether you have to buy for a specific day or not. Guess that is the crux of my question....If I buy tickets on line for the Science or Art Museums do I have to pick a specific date for entry???
Not sure why this post is not showing up on the Chicago thread.

The Art Institute's free days are now the first and second Wednesdays of each month.

You do not have to have a timed ticket for general admission for either the Art Institute or the Museum of Science and Industry. However, if you are going to a special exhibit, taking a tour (such as the boarding tour of the U505 at the MSI) or taking in an Omnimax movie, then - yes - you must choose a specific date and time for each.

When choosing your times, Make sure that you have sufficient time to get to those at the MSI because is one big museum. You cannot just be choosing one each hour after hour. Allow the better part of a day for this museum.