As I read your requirements, you want a recommendation for a graphing calculator for an engineer (or engineering student). An ME, EE or Physics student could actually require a calculator that can support all types of high end math. A Ti 83/84 falls far short of this requirement (although many have used this calculator to graduate as it functions as a decent scientific with a great set of statistical functions, something that is very handy for labs and business majors). And if your professors/school allows a high end graphing calculators with CAS, the following five calculators leave everything else in the dust. I will list them in order of power/features. This order could change as the Ti-Nspire (probably very little left to develop as it has been on the market for nearly a decade) and Hp Prime built in functions are still being developed.
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1st, Hp 50G, 2nd, Ti-89, 3rd, Hp Prime, 4th, Ti-Nspire and 5th, Casio ClassPAD.
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Does this mean you should automatically buy the Hp 50G (and won't be available for much longer)? But it is an RPN/algebraic selectable calculator and if you use it in algebraic mode, the Ti-89 is actually the superior calculator (Hp 50G has bugs in Algebraic or poorly implemented units conversions and other issues when Hp crammed in algebraic support into this line). Both of these calculators are NOT being marketed as both Ti and Hp now push their Nspire and Prime calculators as their high end solutions.
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The Ti-89 is superior to the Hp Prime for many high end math features too. The Hp Prime actually evolved from the Hp 39/40G line of calculators and has several design limitations (single letter variables) that will always make it less useful compared to the Hp 50G or Ti-89.
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The Ti-Nspire is a locked down programming system (to keep cheat sheets from being loaded on and make it more test compliant) as such is also crippled even further than the Hp Prime.
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The ClassPad evolved from the Casio FX 1 and FX 2 CAS line and as such have similar limitations to the Hp Prime. However, the last three have color screens which could clarify differences in graphs when displaying different functions on the same screen.
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My own opinion is that any of these is a bargain to have in your backpack to help with high end math. And where the professors allow these calculators in their class, you might be foolish to use anything less. However, if you're not allowed to use these during exams or quizzes, these then become teaching aids and you might be better off using Math software which can be used in reports and getting an NCEES compliant calculator for your personal use.