What always makes me laugh is I DIY it so i can fix it myself, they make out like a professionally installed system you need a degree to fix it
. They're basically the same the only difference really is a proper baffled tank and properly bolted to the floor for a fitted system.
There really is no right or wrong, DIY if done properly can be just as good. A fitted system takes away the hassle of doing it yourself and generally looks more tidy, especially when it's all in a cabinet or similar.