The answer to this question depends not only the section—math or verbal—but also on the person taking the test. Let’s say you are asking about the verbal section. Well, if you a writer, lit major, avid reader, or just happen to possess a preternatural vocabulary, then I would say take the current test. Maybe. The current verbal relies on vocabulary knowledge but also context. Oftentimes writers are much better at picking up words in context and, from my experience, they dread analogies.
One way to answer the question of whether the new test will be more difficult for you is to take two tests, one from the new ETS book and one from the current GRE book. Better yet, take the PowerPrep test that is downloadable of the ETS website and compare it to your performance on the CD test that comes with the latest PowerPrep test. If you do significantly better on one test then the answer to the question is clear.
Still, there may be a slight hitch to this approach: ultimately, we won’t know what the test actually looks like, until August 1st, 2011, when those first intrepid test-takers become our guinea pigs. My guess is that the adaptive section might be tougher then what the revised ETS book has shown us, both for math and verbal.
Currently the Revised GRE ETS book provides us a few verbal sections, some which are notably harder than others. As to whether this level of difficulty of these questions will be the same on the actual test is difficult to say. Perhaps those around the 90th percentile will see questions far more difficult than those in the book.
Regardless, taking a practice version from both the old and the new GRE books should help you answer this question.