When George Wallace won support with his ideas that "There's not a dime's worth of difference between the Republicans and Democrats," it was clear that he was a challenge to both Humphrey and Nixon. Casting himself as the anti- establishment candidate in a time in American politics when the establishment was being questioned, Wallace proved very skilled in 1968 at making his case.
As he outlined his ideas, I think that Republican Richard Nixon increasingly realized that Wallace was on a path that he himself found desirable. Nixon and other Republican strategists realized that Wallace was positioning himself to rally what Nixon would later call "the silent majority." Actively against the counter- cultural and liberalized establishment of the time period, Wallace appealed to the disenfranchised and politically frustrated White vote. Wallace was articulating a form of the rage that the traditional demographic in American politics had been experiencing. Due to this, Nixon and his handlers recognized that the rise of Wallace would damage their brand more than Humphrey's. Nixon recognized that the audience to whom Wallace was speaking was one that he coveted and felt necessary in galvanizing his constituency against the Democratic challenger. The more Wallace remained in the race, the greater the chance he was taking votes that would have otherwise gone to Nixon. It is here in which Wallace was a greater threat to Nixon than Humphrey.
I think George Wallace's campaign in the 1968 US Presidential Election clearly hurt Nixon more, though considering the fact that Nixon won pretty easily (at least in terms of the electoral count), it doesn't really matter. While Wallace was a Democrat, he ran as a member of the American Independent Party in 1968. What he was most known for was his stance against desegregation, which made him popular with those in the South. He received 46 electoral votes in the election, all which came from southern states, most of which aligned themselves with the Republican Party.
For proof of Wallace's impact, it would be a good exercise to compare the electoral map of 1968 to the previous election of 1964. I've attached links to the two maps below. In 1968, Wallace won Alabama, Arkansas, Georgia, Louisiana, and Mississippi. Taking a look at the 1964 map, one will find that all of those states, with the exception of Arkansas, went to Barry Goldwater, the Republican candidate that year. Considering the Republican candidate won those states in a year in which Johnson took 44 out of 50 states, I think it's safe to say Nixon would have won most of them had Wallace not run.
As a segregationist governor from the South, the states that Wallace won historically favored Republicans, so one can conclude Wallace did more harm to Nixon than Hubert Humphrey.