Your flexibility in taking a different train depends on whether you are booked for an unreserved or reserved service. Trains with unreserved coach seats are relatively rare these days, and all service between Chicago and St. Louis are Lincoln or Texas Eagle trains, which are all-reserve.
As such, you are only allowed to take the train you are ticketed for. To take an earlier or later train, you must change your reservation prior to your scheduled departure, but this is relatively easily done. You can modify it through the mobile app, on the website, with an agent at the station, or over the phone, provided the reservation is associated with an email address, Amtrak Guest Rewards number, or other way that it can be looked up in their system.
As per their current policy, Amtrak does not charge a change fee; however, you are subject to re-faring, so if the new ticket is more expensive than the one you booked, you will need to make up the difference. In the event the new ticket is cheaper, you can receive the difference as an electronic voucher towards future Amtrak travel.
Given this, it would be safer to book the later train, then change to the earlier one if your schedule works out.