This depends on your needs. Check out the differences between the two (from our FAQ page: http://nutribulletblog.com/faq/)
The NutriBullet Pro 900 Series pulverizes and emulsifies fruits and tough vegetables in a similar way as the NutriBullet, but with 50 percent more power, a larger capacity cup, a flip-top to-go lid and the NutriBullet Life Changing Recipes book, a book compiled with tons of user-submitted recipes and ailment-based information to get you Blasting your way toward better health.
The NutriBullet Rx is our most powerful machine yet. With 1700 watts of power, pre-programmed, hands-free extraction, and a soup-heating feature, you can take your nutrition to a whole new level. The NutriBullet Rx has no on/off switch, no plungers, no speed adjustment and no time settings. It’s been pre-programmed to turn at exactly the right RPM, to stop at exactly the right intervals, for exactly the right amount of time – it does the thinking for you!