abdo375: Is this for School or for work? If it is for work, I recommend investing the money and time to learn MathCAD. You will get a lot more out of it then what you will get out of any calculator, IMHO.
If it is for School, then I'd go with the TI-89 as it is easy to use. The pretty print helps out a lot to track input errors such as misplaced parentheses. The HP line of calculators use reserve polish notation. Once learned, you can use it faster then someone using a TI-89 because you won't have to use parentheses. So there is that to consider.