One thing I've notice about the Paris metro is it's all radials and no spokes (bar the line 1) whilst London is all spokes an no radials. Basically meaning that in Londres you have to into town and out again to move around the clockface whilst in Paris any journey usually involves three changes as you swap between the swoops of ever decreasing circles. The Paris metro map is crap too - too many similarly coloured lines and odd curves.
Both have populations just under 10 million 