Different patterns for simulating different bittings, pretty much. Most locks, if rakeable, will rake with a variety of tools so it doesn't really matter. You can use a city rake, snake rake or even half diamond to much the same effect and it doesn't make much difference in terms of time taken either. So in most cases it's personal preference, although some locks are more picky (no pun intended).
For example, if the lock has a dramatic high-low bitting, a city rake won't be able to set all pins. A bogota, on the other hand, will be more likely to succeed. If it has an even or stair step bitting, a city rake or snake rake will work much the same way although in some instances the snake will be more reliable (ie. the lock has a couple of dramatically deeper setting pins grouped together in the middle or towards the back). The traditional snake is also more suitable for padlocks with smaller keyways.
I have an ASSA 600 cylinder here with a stair step bitting which won't rake open with the city rake, although the "W" rake will jiggle it open pretty quickly. You just have to experiment. If you have a good feel for what you're doing you can tell if it's going to rake open, and if you decide to play around with various rakes, you'll probably find one that'll do the job before long. Can't beat the trusty hook pick though
