Hey Guys:

I currently have a TI 84 Plus CE, and I am now also looking to buy a new calculator.

I would like a CAS Calculator that is accepted on the SAT's, but I will not be taking the ACT's, so it will not matter to me if the calculator is banned on the ACT. After doing some research, I have narrowed down the calculator to either the TI 89, TI 89 Titanium , or Ti Nspire CX CAS. I also would like to know which calculator is better for TI Basic and assembly programming.

You don't need anything more than a simple calculator for the SAT. The only reason to get a newer calculator is for upcoming classes. I would recommend the TI 89 Titanium; it is useful in many calculus classes.

Oh yeah, I forgot to mention that I want to be able to use this calculator in an AP calculus class. But it still must be accepted on the SAT. Mateo, you said you recommend the TI 89 Titanium for calculus classes, but what about for TI Basic and Assembly Programming?

The 89T has:
* decent TI-Basic programming, whose abilities can be expanded (and speed can be improved) through ASM programs;
* quite good ASM / C programming: unlike the Nspire series, which is succeeding it, the TI-68k series (to which the 89T belongs) is an open platform, with direct access to assembly, and a good community toolchain (nowadays, GCC4TI). At the time, exam testing mode wasn't a thing, and TI wasn't afraid of people tampering with it and therefore wasn't trying (and repeatedly failing) to lock the calculator down.

However... the 89T became available in 2004, the last OS update for the 89T was released in 2005, and the community of programmers belittled a lot in 2006-2007, without ever recovering. There's been hardly any new TI-68k development activity for years, and that wasn't because of the Nspire series. IOW, the 89T is more than 10 years past its sweet spot, and while I still like that platform, on and for which I worked a lot, I have a hard time recommending the 89T as a new purchase...

While a closed platform on which TI keeps fighting the community, the Nspire CX CAS has more raw CPU power and a slightly better CAS, derived from that of the 89T. Nowadays, new releases of ASM programs on the Nspire series are very infrequent, and there are few developers.

BTW: if your research led you to eliminating the powerful HP Prime, despite its superior CAS and raw CPU power, why ? The fact that basically nobody is interested enough in low-level programming on the Prime ?

Frankly, if you really want to go into low-level development, for learning purposes, there are lots of platforms more useful and cheaper than any calculator, with larger development communities.

The only calculators that have an active developer community are for the (e)z80 calculators.

Oh so yeah, the HP Prime may be accepted on the SAT, but the teachers in my school only allow Texas Instruments Calculators, as I guess the whole class will move quicker if everyone is using just one type of calculator during class, as they won't have to stop and figure out the method of doing something on a different brand of calculator, I guess.

Indeed, that explains why you selected only the 89T or the CX CAS

I really think that other pupils in your class are more likely to buy the aging CX CAS (first introduced in 2011) than the very outdated 89T (2004, but it's basically a variant of the '1998 92+ and 89 with a USB controller + port and a new RTC).
I wonder how TI's going to react to the second-generation of HP Prime calculators, which became available in 2018. The 2013 Prime G1's CPU was already more than twice faster than the 2011 Nspire CX (CAS) 's CPU, and benchmarks show that the Prime G2 is usually 2x-3x faster than the Prime G1, even more in some areas.
Besides the hardware, the Nspire CX's software is in dire need of an upgrade as well: there's still no official form of a Python(-like) language on the Nspire series. Even on the TI-eZ80 series, it's been recently publicly presented in some education-oriented shows, but it can't be downloaded yet, and requires special external hardware anyway.

I mean, I guess I could probably use an HP Prime in class and get away with it, as long as I am not bugging the teacher because I do not know how to use the calculator. Is there a justification for doing so though? You said earlier that the HP Prime has a faster CPU, but for math, does that really matter. Also, is programming better or worse on the Prime compared to the TI-89 Titanium or the TI Nspire CX CAS?

It would be best to be able to check beforehand

The Prime's Basic language, called Prime Programming Language (PPL), is both powerful and fast - again, especially on G2 Primes - and the Prime's OS provides a debugger. There's no assembly programming on Prime calculators.
TI-68k Basic is powerful but not really fast, while Nspire Basic is unusable for multiple classes of programs (for instance, it can't plot individual pixels on the screen), but the much higher raw CPU power makes math Nspire Basic faster than TI-68k Basic.

I did a bit of TI-68k Basic back in the day, but after seeing that the capabilities of ASM extensions to TI-Basic were held back by Basic, I switched to ASM and C. But that over 15 years ago, at a time there was an active TI-68k development community...

Ok, update: I asked the head of the math department, who made this Texas Instrument only calculator policy if I could use the HP Prime in the place of a TI Calculator, and he said that it would be okay as long as I am familiar with it and can use it without stopping the class. I can work my way around it if I buy it, so I guess I can use it. You said it has a debugger on the screen, how does that work? Also, does it come with a software similar to TI Connect for programming on the screen? Also, if I remember correctly, the HP Prime has a touchscreen. Wouldn't that make it count as a cell phone and ban it from use on standardized tests?