Kaplan will provide more CAT's, more comprhensive review abilities (aka, time spent on each problem, number of questions right, number of questions in catagory right, and lists all quesions by easy, medium, and hard). But Kaplan will cost you more. But their online CAT's come with some their really solid online self study course. I used Kaplan and I have nothing but good things. Their CAT tests are accruate and representative of the true test.
At the same time though, I supplemented my studies with the 8 Manhattan Guides, and they were very beneficial. The CAT's aren't quite as accurate (little tougher in quant, little easier in verbal) but for the price you really can't beat them! If you purchase atleast one
Manhattan GMAT book you get all 6 of the CAT tests.
Good luck!