Different students find different tests difficult depending on their aptitude so there is no universal answer to this question. The shortest simplest way to accurately find out which test you are more likely to score is to take a mock exam of both exams. The mock tests estimate almost the same score as the actual test. Having said that, we can compare the two tests side by side to give you an idea of which test you are more likely to ace.
Quant Section: GMAT is Harder than GRE
While I do not think that the GRE Quant section is easy for everyone, I do think that GMAT Quant is less easy, or conversely harder, than the GRE Quant. In my opinion, this is because it is harder to achieve a higher percentile in Quant in GMAT. This, in turn, is because it is taken primarily by business or finance majors so you are competing with the very best and even a few mistakes can lower your percentiles. However, GRE is taken by a diverse pool of candidates many of whom have not studied Maths in the last many years. Therefore, the probability of getting a higher percentile is greater in GRE. So if you are struggling with Maths, the GMAT is not for you.
Verbal Section: It Depends.
If you are a native, the GMAT might be easier for you. This is because there is no hard vocabulary and most natives are less likely to face any problem understanding passages or correcting sentences. In fact, on the easier questions some people can get a lot questions right in the Sentence Correction by relying on their ears or hunches. That said, the harder questions can be really hard and you must rely on rules and not your ears. The non-natives here are at an inherent disadvantage.
From what I have seen, most non-natives are better at memorizing the basic vocabulary and understanding the logical structure of Sentences in Sentence Equivalence/ Text Completion in GRE than at correcting intricate grammatical flaws in convoluted sentences in a second language. Also, in GRE the number of paragraph argument/ Critical reasoning questions is 4 while in GMAT it’s 13 or more and the difficulty level of the latter is significantly higher.
The integrated reasoning section is only on GMAT.
Lastly, GRE is section adaptive and a lot of students find the facility to save a problem for later useful, this is not possible in GMAT