Eurostar

Between Brussels and Paris, no contest. I mean, really, no contest. I have been to Brussels, once , and years ago, I cannot even remember anything memorable( wait, is that the city whos well known tourist site is the peeing boy statue?, if so , I only remember that, LOL )

Paris, well even it you haven't been there I bet you could name off serveral( there are hundreds) sites you have heard about !

Hi Jim,
I have taken the Eurostar many times from London to Brussels, but always on business so I did not get much sightseeing in. I'm not sure that I would recommend spending 6 hours on the train for a day in Brussels. Rather, I would save Brussels for a time when it was on the way (e.g. when visiting Bruges or Amsterdam).

With the recent opening of the St. Pancras station for Chunnel travel in London, the trip to Paris is only about 2 hr, 15 min. If your concern is about the French strikes and not being able to get around Paris once you get there, just take whatever transportation you can find (Taxi?) to the central part of Paris and spend the day walking, if necessary. If you should decide on the London-Brussels daytrip, why not line up train transit to Brugge and spend a few hours there? It would be a very long day, but might be worth the effort. Brugge has plenty to occupy a traveler for 4-6 hours.