Have upcoming flight connecting at ORD (single ticket). Domestic (YYZ preclearance) on AC to International on BR, with transfer from T2 to T5. Between terminal bus vs. ATS, is there one that is quicker?

Also, would there be any need to pick up / drop off checked luggage again? I didn't have to for a recent flight through EWR UA to LX, so wanted to make sure it would be the case at ORD also.