the way an ARB works is by affection the torsional rigidity at the side (i.e. front or back) the bar is fitted. the extra stiffness prevents roll abd therefore prevents the inside from lifting. by doing so increases grip and cornering speed by keeping more tyre in contact with the road for more of the time.
thicker on the front will generally increase understeer characteristics, and fitting rear will increase oversteer characteristics. fitting them both will keep the car neutral. in the Golf, which tends to understeer, it will benefit most from a rear ARB therefore. this is compounded by the fact that you need to drop the front subframe to fit the front ARB, which is both a pig and probably not cheap. hence it is common practice to do the rear ARB only as it's the better value-for-money mod.
which one is better is largely academic. two bars of equal thickness will do exactly the same job, so go on price, weight and convenience. some are adjustable (e.g. Whiteline). i want an Autotech one because they are lighter, but i am by no means an expert on the subject.
