Ok, it’s not me doing the comparing. It’s CityMetric, a division of the New Statesman, that has an article entitled, “Paris has one of the densest metro networks in the world. So we’ve superimposed it on London“.
The article finds these differences,
…. there are some very big holes in the [Parisian] network. Consider the vast gap to the east of the city, which on this map covers Tower Hamlets. No other trains serve that district.
Two thoughts stem from all this. One is the difference in functions performed by these different networks. In Paris, the Metro moves people around the city centre; the RER and Transilien ferry them in from the suburbs.
In London, though, there’s no such division: the Tube plays both roles. The Central line, say, acts like an RER route in the Essex suburbs, but a Metro route in Zone 1.
The other is that this might be one reason why so many Parisian banlieues are depressed: it’s much harder to generate a vibrant economy when there’s no way of getting to a job.
NOTE: Transilien is the equivalent of National Rail, and the RER is the regional express underground rail network serving Paris, it’s suburbs, and the Île-de-France région.