Hi M.!
Now this is a posting that I hsve ALOT of experience with :o) We have had almost EVERY train set in and out of our home over the last 12 years! And with my boys, and my nephews, and ALL the neighbor kids in my home, I know which one's can stand the test of time :o)
The Brio Set is really nice, and pretty durable, too. You really can't go wrong with purchasing one. The only drawback would be price. If your son LOVES the set, then you will try to find all the "cute accessories" for him. That can be VERY pricey with the Thomas/Brio set. BUT... if you decide to purchase the Brio set, your son will be able to learn how to put it together by himself between 3 & 4 years old. Tha'ts a bonus when you buy a train set!
My receommendation would actually be to purchase a GEO TRAX train set. It's interactive for your son's age. The accessories are not expensive at all, so you can keep adding to the set. This set is very durable, and easy to put together. Your son will need help at first, but by age 3-4 he can start putting it together himself.
STORING your train set will be your biggest problem :o) I purchased some (6) large decorative "hat boxes" from Tuesday Morning. They vary in size, so I stack them. It's not the most attractive "display" to have in my living room, but it works for us! Anyway, I can put the trains in one box, the track in the other, etc...
Anyway, good luck with your decision. I hope I could help in some way.
:o) N.