Should I go to Lille?

Go to Paris. To quote an old French king, "Its worth a Mass."

Lille is worth 1-2 days, nice small city and very French.
Paris is 2½ hours from London by Eurostar, Lille is 1½, if you are doing a day trip that gives you 2 whole more hours in France! It is also more compact, everything is within walking distance in Lille (with a small métro system to get you back to the station at the end of the day).
It depends what you want. If you want to tick off a few names (Champs-Élysées, Eiffel Tower, ...) go to Paris. But you will need to plan your day to get around all the places.
Or, if you want to see a real French town, without the rush, have a relaxed meal, just soak it all up, go for Lille.
I would go for Lille. Come back to Paris when you have enough days to really enjoy it.

Thanks Chris, I am leaning toward your point of view.

Some of Lille is ok, some is a bit, well, hmmmm, nearly ok, IMHO.

If you go to the old town you can get some decent macarons and see some cool architecture, but a fair bit of the rest is not as old.

You will be near the birthplace of Paul, but unless you are already a convert to that particularly tasty but expensive French bakery, it won't mean much to you. London is saturated with Paul outlets so if it is new to you, try it in London and save some time. A nice Paul is next to St Pauls Cathedral in London (what a coincidence) and when the weather is decent at 9.30 am on a Sunday morning the outside tables make for a great perch for listening to the changes rung on St Pauls fabulous ring of bells.

But I digress. If you go to the old town of Lille, in fact much of the rest of Lille, be prepared for very narrow pavements (sidewalks) and almost none of the city is pedestrianised.

I like it well enough - I'll be there this week (but not for long) - but for a first timer I'd go for Paris. If you had one more day I'd suggest Brugge (Bruges) which isn't far from Brussels or Antwerpen.

Eurostar is easy, just get there a minimum of half an hour ahead and if you aren't checked in 30 before you won't go, I now always recommend an hour ahead.

Anyway - you will soon be hooked and it won't just be a day or two.

Having just been to Lille in April, I opt for Lille instead of Paris. I know Paris can't be topped. But one day isn't enough to do much for your first trip. There is a nice train ride around Lille from the tourist office that gives you a good overview of the city and some of its history. I went to Brugge too and it was overrun with tourists to the point I couldn't wait to get away. Lille has a vibrant feel, but of a real ville. However, Paris is Paris and even a few hours can be exciting. I'd just be unhappy leaving, feeling I'd seen so little. Lille can easily be done in a day with a lovely lunch included.

I loved Lille when we were there several years ago. The Old Town is striking, there were some beautiful chocolate shops, Palais des Beaux-Arts is a museum you can see easily, and it's not over-run by tourists.

Thanks for the advice everyone. I did in fact go to Lille, and enjoyed it. The day I was scheduled to go was actually the day after the chunnel was closed due to the striking French ferryworkers, and all the chaos that happened thereafter. But everything was running the day after when I went, and I had a nice time. I'd definitely like to go back to Lille for a few more days, and I had to laugh at the Irish pub in the Garde De Lille station "O'Conways." I guess you can't escape Irish pubs, you could probably find one on antarctica.