Arriving at one station; leaving from another in Paris

The ride on Metro line 4 takes 17 minutes. Trains run about every three minutes.

According to worldtaximeter.com, in normal traffic conditions the taxi ride will take around 23 minutes and the fare should be less than €15. You'll also have to wait your turn in line at a station taxi rank.

Juliette,

The Metro will take you a total of ~45 minutes to get from one train to another. There is a fair amount of walking at the Montparnasse Station to get from the Metro to the Trains.

Edwin

Take into account how much luggage you are taking. In many metro stations, you will have to negotiate at least one flight of stairs up/down to street level. Also, what time of day? The trains are crowded in rush hour and it may be difficult to get on with large bags.

To help sort it out:

Walking distance between trains is about the same as walking distance to/from taxis.

Combining what Tim and Chani said: with the metro frequency, if you can't fit easily into the first train, you'll be in the front of the mob for the next one and have no trouble getting on.

Over-sized bags present the same problems in getting to taxis as they do in hiking Montparnasse. If you can comfortably hump everything with two hands, there's not an issue either way.

I have no idea where Tim gets his infinite scoop, but the taxi ride can't be more than twenty minutes and four or five miles. There's no way the fare can approach twenty euros, even in the morning rush, unless you're trying to cram seventeen people with steamer trunks into the cab.

The whole evolution should take about a half hour (maybe a tad more for comfort), no matter how you do it.

I'd go for the three-euro route.

We each have only a RS backpack/

Metro (Line 4, direction Porte d'Orleans, is a staight shot). End of discussion.