The biggest thing I've found that Inkscape won't do that Illustrator will is support (I.e., export images as) proprietary graphics formats like .jpg and .tif. You can only export as .png. This is surmountable by using Paint, but it's an extra step, and you might lose of the resolution options you'd get just by using Illustrator directly. I haven't found this to be a serious impediment, though. Illustrator does have some more advanced options (I think the 3D perspective capability is better in Illustrator), but on the flip side, Inkscape has a more intuitive graphical interface. Having used both extensively, I still think Inkscape is easier to use... even if Illustrator might be more powerful for really detailed work (of which I do very little). Inkscape also loads much faster. In short, if you were making a career of graphics design, Illustrator would probably be worth the investment (especially because you would probably buy the entire Adobe Creative Suite). For the level of what I need, Inkscape is more than sufficient. I wouldn't have ever bought Illustrator if my employer would have let me install it on my work PC. Universities may not have that problem.
My recommendation would be to download Inkscape and learn how to use it. If it sings to you and you really feel the need for the upgrade, splurge for Illustrator. The skills you master in Inkscape are largely transferrable; it'd just be a matter of learning where the buttons are at that point.