Jyeh:
Having taken both tests, following are my observations:
GRE: Verbal of GRE is more vocabulary based. If you have a good vocab and a decent reading background, what you need more is cramming some of the extra words from barrons and taking some practice tests. GRE verbal is manageable. I took GRE more than 4 years back and did extremely well in its verbal part. But it was more due to my very good vocab and some luck rather than my superior language skills at that time.
I dont think RC of gre is difficult..I would say it's of same level as GMAT rc's or even easier..Moroever, you save lot of time in solving antonyms, analogies etc because if you know them...you dont take more than20-25 second per question. This extra time can be used in solving RC's..
Maths of GRE is definately easier than GMAT's maths...Data comparison questions could be bit tricky..but 750+ score is easily gettable..
Also, when I took GRE and even until late 2002, GRE used to have analytical section which was quite challenging and interesting.I still dont know why GRE got rid of that as , according to me, it made the test challenging and Interesting..
GMAT: GMAT's verbal requires not only strong foundation in language but also lots of hard work to tame sentence correction and improve your CR/RC skills. None of the three sections of GMAT verbal is easy. I will definately rate GMAT verbal tougher than GRE verbal
Maths in GMAT is also tougher in the sense that even to score 49, you need excellent skills.
If GRE still had old analytical section included, I might have compared the difficulty level of both the tests as being almost equal..but now with just verbal and maths, I wont keep it anywhere near GMAT.
Rest, it depends upon individual strenghts and weaknesses..
I know some people who did better in GMAT verbal than in GRE verbal..
But these are stray cases..mostly, I have seen people struggling much more with GMAT verbal..
Do let me know if you need any more info..