If you want a cool and very relaxed crossing into Mexico, cross here:
https://goo.gl/maps/an1tv8rwV2chKHaw7 . It is a hand-pulled ferry crossing with very little traffic and a small border town so now issues. You can mostly take county roads to get there. Be sure to get your long-term visitor card and you may have to ask for it since they assume most people crossing there are going to stay withing the border exclusion area. Once in Mexico, I would stick to toll roads over the free road. I would personally avoid "two track" types of roads when possible, just because they are slower, sandier, and are typically near less services. Of course, if riding on one to avoid a major congested area then fine. Have a great trip!