of course we assume that TI calculators are the only ones out there. But HP calculators are much better for engineers. I recommend the HP 50g (look it up on amazon.) I've used the TI83/84, TI89/92, and HP platforms. HP is by far the best. I have 5 or 6 calculators, so I can confirm.
TI 83/84 platform: very ease to use, intuitive. but it's no good as soon as you hit calculus. (i.e. doesn't have a CAS)
TI 89/92/voyage200: almost all the functionality you'll need. It has a CAS, meaning it can do symbolic manipulation and calculus operations. But (imho) it's a very bad interface, and fussing with syntaxes can be annoying, and in short, doing certain things on this platform are much more complicated then they need to be.
HP 50g: all the functionality of the TI89, and then some. But it's also very sensible to use, once you get the hang of it it's as user-friendly as the ti84, but has all the features you would associate with the ti89. Once you start off with an hp calculator, you figure that it does things exactly as they should be done, whereas the ti89 made things unnecessarily intricate. several features in particular (equation writer, and index of how to use commands, and others) make it very worthwhile. Downsides: you'll probably be only one of few using this, so it's harder to get help.
But hp calcs also have a very wide database of actual programs for the calculator suited for various engineering tasks. (hpcalc.org) The programs available for ti calc are pretty much just games. Amazon.com: HP 50g Graphing Calculator (F2229AA#ABA): Electronics