Me and my wife have a ninja and my mother has a vitamix. She loves it and recommends it to everyone. I use hers when I'm visiting, so here's my comparison, if interested:
The ninja we got came with the personal cup attachment, which was the main thing I was interested in to make protein shakes. We've had it replaced twice and they've never lasted for more than a few months. So now i make my shakes 2 at a time in the regular pitcher. The ninja we got is digital. It's sort of a time consuming process to make a shake with ninja. The vitamix is way faster and simpler. You don't have to deal with the annoying blade assembly, it's easier to add something while you're mixing, it's easier to clean. It's also a bit quieter. In terms of power, they are pretty close, but I'll occasionally add kale or spinach to my shakes and the vitamix mixes them noticeably more consistently. I would probably upgrade to a vitamix if shakes weren't literally the only thing we used it for. Ninja's still a pretty good blender. I suspect the personal cup for the vitamix is probably just as shoddy as most other personal cups, but the vitamix pitcher is easier to clean than the ninja.
One thing I have noticed is that my shakes come out with more volume after I make them in the vitamix. I put them in bottles after I make them and when I use the vitamix there will be too much to fit into the bottles. For me that's not a positive.
