1 3/8" is standard for interior doors. We build from $700K up to around $2M homes and never do larger than 1 3/8". If you want something more substantial, do solid core. That being said, solid core does typically double the cost in a lot of situations. Between the cost of the door and additional labor costs to the finish carpenter as they are more difficult to set as they are heavier. I would agree with your contractor and door supplier with regards to using a more economical door. Especially if you are doing paint grade. Molded doors (masonite, jeld-wen, etc all make them) paint very well, are budget friendly and are utilized in 90% plus of new home construction.

That being said, Simpson does make molded doors as well. They do tend to be a bit more and most door dealers use a more local option that stocks the molded door of their preference. Chances are, the dealer has better pricing for a simpson alternative which should be just fine.