There are three classes in Thalys service: Premium (1st class with catering), Comfort (1st class without catering) and Standard (2nd class). There are some more differences in seat configuration, Wi-Fi, cancellation policy and so on according to the travel classes, so I recommend you to have a look at their website before purchase (find ‘Info& Service’). All the seats are reserved in Thalys, so you have to buy and collect tickets at the boarding station.
I think that Comfort Class provides high cost-performance, and they seem to be fully-booked earlier. For example, when I once looked for the ticket from Paris to Rotterdam a week before my travel, Premium (discounted) was cheaper than Comfort and Srandard (ordinary fare). The ticket is bookable up to 4 months in advance and the fare is going up as time goes along, so you should make a booking soon if you have already had a plan.
Train: Thalys 9309 (High-speed railway)
Route: Paris Nord to Rotterdam Centraal
Time: 7:15- 10:12
Seat& Class: Coach 11, Seat 61 (Premium/ 1st Class)
Thalys Lounge (Paris Nord)
Premium class passengers or Thalys TheCard members have access to Thalys Lounge (Conditions and opening hours). Thalys has more than 20 destinations in western Europe, and they provide lounge service at Paris Nord, Brussels-Midi, Germany, and Netherlands’ stops. In the case of Paris-Nord, Thalys Lounge is located at the south-west of the station (5 minutes on foot).
Paris North Railway Station (Gare du Nord)
The lighting inside the station was so picturesque that I could not help but release the shutters. But be careful when you take Eurostar from Paris Nord. There are always so many passengers in line, waiting for the security check and passport control to enter the UK, which does not conclude the Schengen Agreement with other European nations.
Keep in mind that Eurostar trains may delay depending on the size of the crowd. Therefore, you have to try to reach the station earlier in the case of no-deal BREXIT, whose confused situation might cause more crowds and delays (Actually, French customs officials have already launched strikes, saying that they would overload with work). At least, currently, Standard Premier and Standard passengers have to arrive at the station 45-60 minutes before departure (Except for Business Premier of 10-15 minutes). You can check the conditions in detail here.
The seat configuration (Premium& Comfort: 1+2, Standard: 2+2) and the less crowded cabin provide us with a comfortable environment, but the catering meal (only available in Premium) is not worth the fare. Premium is convenient when using Thalys for business (flexible time ticket etc…), but say you want a little more comfort during the trip, Comfort Class should be better.
By the way, you cannot select a specific seat during the booking process at the moment (only preference is selectable). In addition, you cannot tell whether your seat is forward-looking or backwards-looking. This is because the direction of wagons varies from one carriage to another. Nevertheless, you can find your seat location from this seat map.