I will be traveling from Paris to London via Eurostar in mid-September with my wife and son. I notice that there are three categories of seats (economy, comfort and premier). I find the cost of the premier seats a bit expensive, so my choice will be between the economy and comfort. Can anyone tell me what is the difference in accommodations between the economy and the comfort? Also, how soon in advance is it recommended that I purchase the tickets? How is the luggage handled? Any other useful information will be appreciated.