The trains heading to the north leave at xx:44 after each hour. You will take an Belgian Intercity for your first leg. You will then change onto the faster fancier expensive Thalys at Antwerp or the international Intercity at Mechelen.
The Intercity to Antwerp -Thalys to Rotterdam - Intercity to Utrecht trip is overall 2:14 whereas the Intercity to Mechelen - Intercity to Rotterdam - Intercity to Utrecht is 2:59.
They both start with the same train from the airport.
The Thalys second class is probably nearly as good for space as flying First Class on many airlines, with 2+2 seating and power points, etc. The only thing it doesn't have is food service to the seat. First Class free (included) food is pretty expensive if that is the only reason you go 1st (1+2 seating) in Thalys. Reservations, which can be made just ahead if you wish but are very expensive, are mandatory on Thalys; they don't play well with passes.
See the descriptions and photos at the excellent The Man in Seat 61 website.
The intercity trains have both table and airline type seating too, but are older, with many fewer (but eminently liveable) and are much much cheaper and the ticket can be bought just before you board. Reservations are not possible.
There is a photo of upper deck second class seats on a Dutch double decker Intercity train at This page at Seat 61
I personally never travel First Class on European trains unless somebody gives it to me.
If you buy Thalys tickets on the day you will pay top dollar for them anyway so maybe as long as you are splashing the cash you might prefer even more room and a meal at your seat and buy First.
In any event you will have bookends of Intercity trains, so you might want fewer neighbors (probably business people trying to work and foreign tourists using first class passes) especially as in your other thread you said you will have lots of luggage.
I'd advise trying to make either the 8:44 or 9:44 run. If you wait until the 10:44 one it is nowhere as easy as I have described as you have additional close timed changes (no fun with luggage, especially as you said lots of luggage) and gets you to Utrecht later than the 11:44 from Brussels airport.