You should fly AA. They will protect you across tickets in the case of delays.
You don't say where you're coming from, but if you're on an E175, it will have similar reliability as mainline.
The big issue at ORD in the spring (which taking either regional or mainline doesn't help with) is ground stops due to lightning. Generally, if there is going to be a delay due to weather in the spring, it's everybody (in my experience).