Well, a calculator is much more handy if you have to do only quick and simple calculations.
You can bring it always with you and, unless you use a subnotebook, is much lighter than a notebook, moreover is always ready to use, instead if you want to use a pc you have to turn it on and wait a little.
If I was in your situation, I would buy a cheaper calculator (no more than 60 euros) and a Maple student license.
Yes, SAGE is free, it is a good and reliable software, it introduces you to Python programming language and probably it has all you need for your current studies, so I recommend you to try it (and also maxima (it is included in SAGE, but it is a standalone program), octave, scilab and gnuplot), but Maple has better capabilities (beginning from plotting and far more), there are some packages dedicated to students and there are a lot of guides and worksheets that you can download and that can help you to better understand some mathematical concepts: most of them are free, but there are also some quite good guides that you can buy on Maple web store.
Maple has a nicer interface and you can easily do a lot of calculations just clicking on menus: I don't like "Clickable Calculus (TM)" very much and I suggest you to learn to use command line, however this feature allows you to use from the very beginning most of the power of Maple (at least up to entry undergraduate level) and sometimes it can be very confortable.
Moreover, Maple has also good typesetting capabilities: I suggest you to learn Latex, but sometimes you can save time using Maple for some tasks.
About the documentation, in addition to what Alec has already written, I suggest you to give a look at the useful tutorials and worksheets that you can find in the application and student center on Maplesoft site.
Have a nice day.