I have bought both MGMAT and Veritas courses, on top of Knewton, PR, and Kaplan. So, I have taken all and am taking all of their CATs. It cost me quite a bit of money because none of them sells CATs alone, and when I bought MGMAT they weren't selling CATs as a standalone product yet.
It's hard to say which CATs are harder because you can be more familiar with one topic than another, and there is not a perfect distribution of topics in all of these CATs. So, while some people, for example, might be better with parallel reasoning (CR) might say Veritas, for example, is harder others who are not all that great with parallel reasoning might say the opposite.
If I'm going with overall scores, then I will rank these CATs in this order:
1. Manhattan (great for quant practice and for getting your verbal basics down)
2. Veritas (great for testing harder verbal concepts, which you may or may not see on real test)
3. Knewton (great for verbal and quant. content is good, but the delivery of the questions is lacking)
4. Kaplan (all around good content. if you're aiming for 650 to 680, this will be enough for you)
5. GMAT Prep from GMAC
6. PrincetonReview (not the best product overall, in my opinion. While i was scoring low 600s with MGMAT, I was getting low 700s with them)
If there is just one place you want to spend your money, i recommend MGMAT. They also allow you to reset your CATs so you can either get more difficult questions as you get better or just to work on your timing. Lots of companies like Knewton tell you the "downside" to repeating CATs, but that's because their pool of questions is not big enough yet and indeed if you were to rest these CATs you're practically going to see the same questions. Plus, I suspect they don't want to expose all of their questions in the pool. Kaplan, by the way, allows you to reset as well.