I have a good friend who uses the nutribullet regularly, and we just purchased one for my mother-in-law. They are both in love, although it generate quite a bit of noise. I really don't have any experience with the ninja so I can't really help out more than that. Good luck.
I did some research a while back and I ended up getting the nutri ninja. You get the same 900 watt motor for only $99 vs $129 for the nutribullet pro. The ninja also has a design where you don't have to worry about the seal on the cup causing leaks, etc.
I have had ours for about three months, used multiple times daily, and it has been a great addition to the kitchen.
If you want it a bit cheaper, keep an eye our for bed bath and beyond coupons (and if you use discover card - you can use your cash back rewards for BB&B gift cards).