Do let us know what you finally decide and how you get on - always interesting.
I tried the Lille and TGV option on Saturday; worked OK but to honest the whole process was rather cumbersome. Flight to Lille was quick and on arrival a car met us at the general aviation apron and took us to the main terminal for customs etc. To my surprise there weren't any taxis outside (I guess we must have been in a bit of a lull of commercial flights) so had to wait about 15-20 mins for a taxi to come.
TGV then took about an hour into Gare du Nord. On paper it doesn't sound that bad but whenever you have multiple modes of transport involved; taxi, train etc one loses little bits of time in a cumulative fashion. The TGV is fast (ironically much faster than the train from Beauvais to Paris despite being three times the distance!) but the service is only hourly so unless you time things well there will inevitably be a wait for it.
I think Toussus might have been quicker because a stop at Deauville or something could have been done very quickly and once there we could have taken a taxi to a station and then quick train into the centre.