You are correct and it is more likely that our liberal friends up north would allow radical muslims to enter their country (through an airport) than our corrupt neighbor to the south. I also think the drug cartels in Mexico would'nt be enthusiastic about sneaking muslim terrorist over the southern border because that would be VERY BAD for their normal business in the long run but they could probably blend in with the rest of the horde. The price of dope rose considerably after 9/11 with the tightened border security and screening.
The northern border would be much more difficult to fence but they are not trying to immigrate to our country (outside of Disneyland/World visits).
Canadian pot smuggling is HUGH and series business; so is illegal crossings of Pakistanis, Arab Muslims, terrorists, and European illegals.
You DO need to read up on this. :-)
The northern border would be much more difficult to fence