To add to the good advice Laura has given you, the seats are all about the same. Doesn't matter. You will need to be at the London train station about 45 minutes to an hour beforehand. You go through a security check, then French passport control, then to the waiting area. If you don't allow enough time to get to the station, you may miss the train, since the screening usually closes some time before the train leaves. There are shops both before and after security/passport control.
When you arrive in Paris, you may want to take the metro, especially if you don't have a lot of luggage. There is often a long tine for taxis at the train station and traffic can be very heavy during the day. Of course, the metro may mean lugging your stuff up a flight of stairs. . . .