I've also heard that if someone was to use apt-get or aptitude, that they should stick to one and not mix. Is this so?
I mixed them for a long time and never got any problem with it :3
one of the reason could be because any package installed/removed with with apt-get, aptitude want to revert it. you can try it by yourself
1. run aptitude on your terminal and cancel his pending actions. ('sudo aptitude' - actions - cancel pending actions)
2. install random small package with apt-get
3. run aptitude again. check the pending action (press g). you'll see that aptitude want to remove that package.
nevertheless, it's just a small matter and the aptitude behaviour can be safely ignored (simply cancel the pending action just like we did above). the real problem is that ,as described by z, apt-get and aptitude have different way to resolve broken package. on my experience, this isn't a big issue on debian stable/testing. apt-get and aptitude on most cases gives similar result. however, on debian sid, the result could be quite huge. personally I prefer aptitude over apt-get for installing/removing packages for two reason.
1. it could autoremove unneeded package.
2. apt-get only gives single solution while aptitude gives me more than one solution so that I could choose the best for my system.