Comparison between the TI 89 and the TI Nspire

Other comparisons on the web mostly compare the two from an academic standpoint. This comparison will be more in depth, as it is a comparison from the viewpoint of a programmer. The Nspire being compared is a TI-Nspire CX II CAS and the TI-89 is a HW4 TI-89 Titanium running AMS 3.10. (Patched with TIOSMod+AMSPatch to improve speed, get more flash, and nuke the localization, HW3patch to allow TSR programs, and FlashAppy to allow unsigned flash applications.) Assembly language benchmarks are nonexistent because at the time of these tests, Ndless was not available for the CX II CAS. (And so it couldn't run native code.)
We'll start with a direct comparison of the specifications:

Just looking at the raw specifications, the Nspire seems assured of a victory. Wrong. When we tested the Nspire, several things jumped out at us, and the ancient TI-89 was still able to put up a fight.

First of all, the interface seems modeled after the TI-89. That's great, since the TI-89 had a great UI. It works, but the menus are obviously made for use with the touchpad. The touchpad is, in theory, a great idea, but in practice it is next to useless. It's not sensitive enough, requiring an ungodly amount of force to move the cursor, and it's pretty difficult to avoid the arrow keys when you're pushing down. It's also too small, requiring you to move your finger multiple times.

We looked at the "MathPrint" formatting, and it's what you would expect. It makes it a little easier to understand than the TI-89, but the placement of the touchpad and arrow keys makes it hard to navigate, especially if you don't like to have to dodge the touchpad while navigating around. The CAS is similar, if not the same, and its speed has been improved quite a bit.

Next, we looked at the programming. The processor is 25 times faster on the Nspire, so we expected the BASIC execution speed to be improved as well. What we found is that the language, while similar in structure, executes at about the same speed as on the TI-89. Lua was only slightly faster.

Yet another difference was in the most important part of programming: assembly. Texas Instruments opened up the TI-89 long ago, and countless programs have been written for it. It has a C toolchain, based off of GCC, and a solid assembler. It supports native code, and there is plenty of documentation for everything from ROM calls to IO ports. The Nspire is different. It does have nice tools, and the vast amount of memory seems like a programmer's dream, but there's a catch. From the moment the Nspire was released, TI has opposed every effort to allow arbitrary code execution. Every version of the NSpire OS has a corresponding version of an exploit called Ndless. TI patches Ndless in every OS release, making NSpire programs volatile and prone to breakage in between OS releases. Another advantage the 89 has over the Nspire is its instruction set: 68000 Assembly is much nicer than ARM Assembly. You get more registers, more instructions (so you write less code, which is a big plus in assembly), and all sorts of little programmer friendly goodies. I'm not sure why you would program in assembly for the 89, though, since the C toolchain can produce better code than you can.

The TI-89 has had its own share of breakages, but those are all resolved now that the OS has remained the same for well over a decade. All the aforementioned problems with the Nspire don't exist, since the 89 is an open platform.

In summary, we would recommend staying away from the TI-Nspire if you intend to do serious programming. If you're just trying to find the best calculator for school, it might be okay, but if you're writing programs, you have to be extremely careful with the versioning. In spite of all its benefits, TI's attempt (while laughable) to lock it down kills it as a development platform.
Other comparisons recommend the Nspire, but our verdict is:
Stick with the TI 89.