Another important difference is that the 1:500k has the maximum elevation figures on it. Your instructor may want you to use those for flight planning.
And in the Nav exam, you'll be given a 1:500k one, so you might as well get used to using it.
There is also the problem that the place names are different between the two maps. Some appearing on one, but not the other, and you'll probably find that your local ATC will use the ones on the 1:500k map.
I'd recommend getting the 1:500k chart, and if you feel the need, get the digital 1:250k one and print out any areas of interest to you on A5 so you can stick them in your kneeboard.