They are entirely different tests, so the results cannot be compared. You can have the cousins take one or both types of test.

They serve different purposes. The Y-DNA tests (FTDNA and Ancestry) follow the all-male lineage, as do surnames for most of the lineages we study. The results (haplotypes)for living males will be similar to their male-line ancestors, since the Y-DNA is passed from father to son with only infrequent mutations. Thus, we can group males with the same surname (or variant) who might share a common ancestor and eliminate those who cannot. This is especially useful for checking the validity of family trees, which normally follow the surname.

Autosomal tests (23andMe, Family Finder at FTDNA and AncestryDNA) test all the lineages in a pedigree. Because the number of ancestors contributing DNA increases rapidly as we go back in time, the matches are accurate only back about 5 generations.

The firms post lists of close matches. Researchers can contact them to see if they can identify a common ancestor. However, the response rate at 23andMe is low, because so many tested for the health information. Even though it has a smaller database, the number of responses at FTDNA appears to be about the same. It also has better genealogical tools.

He can save some money by transferring his 23andMe results to FTDNA, but, with their new pricing, the savings might not be worth the trouble. However, it is a good idea to be in all three databases to maximize the number of matches.