I find the Manhattan maths guides to be more concise and they have more quick solving techniques. I also had the opputunity of attending one free lecture from either place and found the manhattan one to be more informational. however, please note that this is only for the quant portion.

With regards to the verbal portion i find that both are equally informative however the qs tend to be more difficult in the kaplan than the manhattan gmat which is obviously better since it will prepare you for the hard questions.

the manhattan gmat CAT exams also has this neat feature that show that mistakes you have made and categorizes them(for example rate or ratio etc).

i guess at the end of the day, in my opinion, its boils what area you think you are weaker at, quant or verbal, and then enroll in tha course that will suit you better.

All the best....