Answer:
This is an interesting question. A few years ago cordless drills couldn't compete with a 240V version but today I think the choice is really down to how much freedom you want with the tool. The battery power on the good quality cordless tools is fantastic. They last for a long time under heavy loads and have plenty of power to even drill into concrete.
Just decide what application you want your drill for, if it's the odd job around the house, drilling into softwood and screwdriver operations, then a 9.6V battery version will probably be enough. If you think you want a tool that will meet every possibility and last a lifetime then go for an 18V model, but which ever do choose make sure it's a quality brand. You only get what you pay for has never been a truer statement when it comes to tools.