Which is stronger: the Intel Core i3 (8th Generation) or the Intel Core i7 (2nd Generation)
From a purely technical perspective, the Intel i7 from the second generation is stronger.
Around 6th gen, Intel decided, for whatever reason, that they would make the substrate (the part that goes between the silicon and the socket) thinner. All future processors (not used for HPC purposes) inherited this thinner substrate. On that basis alone, the 2nd Gen i7 is stronger than the 8th gen i3.
On top on the substrate differences, there’s also the use of solder to transfer the heat from the silicon to the integrated heat spreader. 3rd gen and later used thermal paste, while 2nd gen used solder; which makes it more rigid in that area as well, as solder is metal compared to paste, which is not.
Undeniably, if you had to use either one or the other as a building material, the i7 would be a better choice.
Addendum, June 11, 2020: With the arrival of the 10th generation, the latest i9s and i7s have reclaimed the title of Strongest Intel Processor from the 2nd generation. While the substrate remains thin, they use solder instead of the paste (something shared with the 9th generation). However, the 10th generation has thinner silicon as compared to the 9th generation (to aid in thermal performance). To compensate for the thinner silicon, the heat spreader is that much thicker as compared to all older processors, resulting in significantly better structural rigidity overall. Just don’t push too hard on the exposed substrate, that’s still thin.