It depends on your goals completely. the Rarity factor goes to the MKIII, its much easier to mod, but more expensive once you go past BPU level. see which car you enjoy driving more, which style, interior, exterior,...etc you like more. after all its you who will be driving it and no matter what anyone else thinks it wont matter since they wont be paying or driving your car.
I personally believe the looks of the MKIII are the best for cars that age, when I was in the market for a project car I could have easily picked a MKIV, call me crazy but I went with a MKIII, most sane people wont agree with my decision, but it was mine to make and I don't regret it. it will be the same situation for you, which car you like most, which one you see your self modding and dropping money on to keep for years without looking back and saying WTF did i do.
I personally believe the looks of the MKIII are the best for cars that age, when I was in the market for a project car I could have easily picked a MKIV, call me crazy but I went with a MKIII, most sane people wont agree with my decision, but it was mine to make and I don't regret it. it will be the same situation for you, which car you like most, which one you see your self modding and dropping money on to keep for years without looking back and saying WTF did i do.