There are 2 types of TI-89 Graphing Calculators:
The original TI-89 (1998) and the most recent TI-89 Titanium (2004).
The TI-89 Titanium is the better of the 2 TI-89 calculators: It has more memory (4MB of Flash Memory and 256KB of additional Memory RAM),
the faster Motorola 68000 processor and the modern LCD Dot Diplay. It uses the OS 3.10.
The best selection and best prices for Texas Instruments TI-89 can be found at Amazon.com and Walmart.
At Amazon you have the option of either buying a new TI-89 Calculator or to buy a used/renewed one.
The short answer is: It depends! Read the advantages for each calculator below.
5 Reasons in favor of the TI-89:
1) The TI-89 has a Computer Algebra System (CAS) which is what you need in Calculus, PreCalculus, Algebra, Engineering classes to solve Equations, find Derivatives and Integrals, etc.
The TI-84 does not have CAS and cannot help in that regard.
2) Stylish silver/black design, computer connectivity using the free TI-Connect Software.
3) The price is right. Buy a used TI-89 Titanium for $50 a less, new for $130.
4) Many preloaded Apps. Free Games can be downloaded at https://www.ti89.com/games.php .
5) The TI89 allows for STEP BY STEP Apps for Algebra, Calculus, Physics, Chemistry, etc to be downloaded at www.TI89.com
5 Reasons in favor of the TI-84 Plus CE:
1) The TI-84 is just easy to use. Everybody loves the simplicity and user friendliness.
2) Colorful screen and design. Cannot beat that! Computer connectivity using the free TI-Connect CE Software.
3) Use your TI-84 Plus CE to Code in Python on the Go. Nothing beats that.
4) The TI-84 has a greater memory which allows for many APPS, GAMES and more.
5) Charge your TI 84 like a cell phone. No need to replace batteries anymore.
You may downlad the free TI-89 Titanium Manual (PDF) here:
TI 89 Calculator Manual