While your view of Cruz on the border issue is understandable (i.e. I can see why someone would believe that) He is PA most folks see it the other way with Trump being the stronger border guy.
Why?
1) Trump was the one that made this an issue, he drove it, was attacked for it and did not back down. He made it the issue it is , all the other candidates wanted to just look the other way.
2) No doubt Cruz was and is strong on the border, but early in the campaign, he did not want to talk about what to do with the illegal folks here. His stock answer was “ the American people do not want to have that conversation until we secure the border”. He in PA he is viewed, rightly or wrongly as having only gotten off that stance when Trumps positives forced him too.
It Would also be fair to conclude, as some have that Cruz only came out for ejecting the illegals once he saw it was a popular stance. Either way he does not look as strong as Trump, at least to he majority of primary voters.
3) Cruz was on record in 2013 to favoring more H1B visas and did not change that stance until 2015. Folks here in PA see that as a method for wages to be depressed.
4) Trump is the only one who came out against Muslim immigration. Rightly or wrongly a lot of folks support that. Cruz does not.
The reality is here is PA the ratio of roughly 4.5 Trump supporters to 2.5 Cruz supporter seems to prevail. As immigration and Trade nationalism seem to be the 2 big issues, most folks around these parts see Trump as stronger on this issue.
Cruz supports a ban on Muslim immigration until the vetting process is verifiable. This is exactly the same position as Trump.