That's actually quite the controversial question.
Windows, Linux (Ubuntu), and Mac all have their strengths and weaknesses. As far as security is concerned, each operating system if sitting standalone is essentially completely secure. In a hacking contest done this same time last year, they put each system to the test. Hackers were unable to hack into any of the systems if they were just sitting idle without any programs running.
The weak points that make a computer less secure are actually the programs running on them. Internet browsers are the most often attacked out of any of them. Apple uses Safari by default, and in the contest it was the first to be hacked using a weakness in the browser. However since then they have updated it and fixed the security leak. [click to read about the contest]
So it isn't really a substantial hit against Mac.
Honestly as long as you always keep your computer system AND
on it fully updated you'll be relatively safe. A good anti-virus
program is a must-have if you have Windows
, there are a few free options or you can purchase one but you'll definitely need one with Windows.
A lot of people claim that a Mac is "virus-free
" which is mostly true, the only reported cases of infections were from directly downloading infected files or inheriting them from a Windows machine on the network.
So if you ask me, a 9th grade girl that will probably just be using the machine for schoolwork.. well I'd say a Mac is your winning bet.
Hope that helps out a little, keep us posted if you have any more questions.