Well, as the headline indicates: Which one should I purchase? It needs to be able to differentiate, integrate etc. (also functions with more than one variable) and if it's able to draw circles, ellipses, 3D-images or similar, then that would be a great plus.

I have a TI-84.

Thanks in advance for your answers.

Well, I have had my Ti-Nspire CAS for a few months now, and here is what I've concluded.

The display on the Nspire is WAY better than on the 89, and It's a lot more organized and easier to use with Integrals, derivatives, sums, products, limits, etc. because you can type in all of those things directly into the calculator exactly how it looks on the paper, and it's easier to see equations when you're typing it into the calculator because for some things, it puts it into pretty print while you're typing it in. And, the statistics that an Nspire can do is by far the best there is, way better than an 89. Also, you can connect the calculator to the computer and install updates for it directly(I don't know if 89 or voyage 200 can do this).

But, what I don't like is mainly the graphing capabilities. It is completely different than the way the 89 and 83/84 graphs, because you type in your function below the actual graph, and it pastes what the function equation is right next to the graph also. And how it traces the graph is kind of annoying, because you as it goes along the graph, it tells you the max/mins that there are, and intercepts, etc. I liked how the 83 did it with the calc command that you go left bound and right bound, and it gives it to you. Also, I can't stand how it graphs Parametric Equations, and Polar Equations, because I would prefer how you can set up the window in another screen. I particularly liked how you could set up the tstep and θstep on an 83/84

From what I can see, the Nspire cannot graph 3D or Implicitly. I am not sure the 89 can, but I am pretty sure the Voyage 200 can.

so overall, if you dont want to read my ramblings
Pros:
Display is neat
Top of the line statistics
can save documents
the ability to update the calculator
higher resolution display
I also LOVE the ability to define functions. (89 and voyage 200 can also)
Cons:
Graphing (it still can graph functions nicely)
more difficult to calculate max/mins/intercepts/area directly from the graph.
no 3D/Implicit graphing

Im not sure, if TI will make an update so that it can graph 3D and Implicit equations, but it IS possible. Only time will tell.