TI89 is definitely better in almost all aspects than a 83+/84. Though I am used to the 83/83+ interface and using a 89 effectively would take a little bit to get used to, but it's certainly more powerful.
However, I did just fine with the 83+ since I got it in 9th grade and now I'm in my 4th year of college. My freshman year I had Calc 1/Calc 2 which though you could use an 89 in, if you didn't understand the Calc you would never pass the tests. Got A's in both of those. Then in my 2nd year I had Linear Alg and Differential Eqns. Got a B and A in those, respectively. My 83+ was adequate for Linear Systems and Signals, Circuits, as well as other classes so far.
So really, not having a 89 makes you actually learn the math. I can't believe some of the simple Algebra techniques I barely knew when it came to some college courses, but with 20 minutes of practice and I could do it fine.
So I guess with the fairly small $ difference, the TI89 would ensure you have an adequate calculator for your whole college career. But honestly unless you are in Engineering or Science majors, a 84 would do just about everything required, and even then my 83+ still works good enough for me. Plus if you only have a 89, what about tests that don't allow it? None of my college courses ever restricted it (except QWERTY ones), but seems that people say they often can be not allowed, so you don't want to be stuck without a calculator.