[elcapitan]

They can both be helpful. The grid system is great for getting from coordinate (a,b) to (c,d). Much better than a natural city core as most older european cities have (and I mean old in the european meaning, i.e. >1000 years). But grids are extremely bad for mental mapping, as they are virtually featureless. So to remember your paths in a city without the coordinate system, it is much more helpful if the place has curves and outstanding features and little weirdnesses and so on (you can have outstanding sites along the way on a grid as well obviously, but because the streets are all just long tunnels, you will almost never see anything of it except for extremely tall parts like the mentioned mountains outside, or very high buildings).

[EGreg]

For me grids are way better for mental mapping!

Each street and intersection has different buildings, stores and vibe.