I took the GRE (general test) last year and that was pretty tough. I have never take the GMAT, however, I have a few friends that have taken it and in terms of difficulty I would give the edge to the GMAT. I think it quantitative sections are more difficult than those of the GRE.
I have taken them both - I found the GRE easier, but it depends on the person. I would get both books with practice tests, and see how you do on each and what percentile it would put you in.
