De rien.
Normally a taxi would beat the Métro, especially at night — but in rush hour traffic there's always a danger that even the zippiest driver could end up stuck in a bottleneck. Nothing worse than watching those minutes tick away as you inch forward at a snail's pace. At least with the Métro the trains come every 4-5 minutes at rush hour and you know that once you step on the train you have X minutes until arrival at the station you're getting off at. It will be tight, but with a bit of luck you should be able to make it in 25-30 minutes.
Is it too late to amend your train booking? It would take the stress out of it if you had an extra half-hour to play with, and I imagine it would certainly be cheaper (if not free, depending on your ticket and how far away your travel date is) than having to transfer/rebook after missing the 6pm departure ...not to be putting a jinx on you!