In my opinion, iBT is the hardest but the most useful one at the same time since it is an integrated test and it requires essential skills that you need for graduate school.
I took iBt two years ago in November when CBT had just been replaced by iBT. I was quite worried because I had been preparing for CBT for a while. I've got only 10-15 days to prepare for iBT at that time. Listening and reading are about the same and you should be able to get high score on these parts if you are familiar with the pattern of the question. I did pretty good on those parts as well as on writing because I took GRE exam before this test. I still couldn't believe I did well on writing ^__^. However, I did not do well on speaking part. I passed after all.
So I think you may need to see the website of the school you are applying that which one they require. I believe most of them require iBT and CBT. I don't see any PBT now. Check with the school requirement na krab. If most of the school you are applying require iBT, then you should prepare for iBT krab. Don't worry I think if you allow yourself enough time to be familiar with the test and the pattern of questions, you should get over 80 scores for sure. Practice makes perfect krab. You may try to take a diagnostic test first to see how well you do on the test. If you think you don't have enough time to prepare for iBT and do better on CBT, then go for CBT na krab. But I still encourage you to take iBT na krab. It is very useful indeed. I hope this helps
wink