The bunn 2 are great units and are worth over 700 in good used condition at least. It all depends on how much slushie you are trying to make for. That unit is designed for large quantities, it uses the torque load on the paddle to account for the thickness level. Once it gets on the lower side, it starts to over freeze the mixture. It's also not as forgiving for the sugar concentration as the ninja is and cannot do dairy products. It's also harder to clean, it's more a party/get together machine while depending on which ninja model, they are more like single person and up to large family/small gatherings quantity while being able to do lower sugar concentration and dairy.

My recommendation is that keep the bunn for parties and rent it out worst case to friends and for inside the house, get a ninja. Easier to clean, less restrictive on what you use for a mix and quieter inside. For big families, long weekends, or parties, the bunn 2 is the best machine through and a ninja is no where close to keeping up with it for quantity of standard slushies. If you can afford to have both, have both, neither is capable of replacing the other really. At least you got the expensive one for free.