I take Rt 5 through Mexicali to Rt 4 to Rt 40 at Coahuila city. Then 40 south to get on the Costal highway. Only tricky spot is where 4 ends at Coahuila. Go left over the tracks pass the OXXO on the left side and end up at the T on 40. Rt 4 is a little picturesque through several small ejido towns but the road is good all the way.
Rt 2 through San Luis to the south bypass I found confusing. Poor signage.
Have not driven it in many years but 2 to Sonoita is a narrow killer road with convoys of big trucks. No trucks on the other route, well maybe one or two but not many.