I would think the solid MDF MIGHT be more stable over time due to it being 100% frabricated material. The poplar one might be a little heavier feeling, which you might like. I know I like a heavier feeling door. They usually close nicer as well. (or slam really hard when angry)

"I would think the solid MDF MIGHT be more stable over time due to it being 100% frabricated material."

Doors and furniture are made using panels to reduce the effects of the wood expanding and shrinking. Each panel has clearance in the grooves holding it to allow the panel to move separately without altering the size of the door.

MDF is only moderately stable, and changes size in ALL directions (unlike solid wood that only changes in two of three axis).

"The poplar one might be a little heavier feeling, which you might like. I know I like a heavier feeling door. They usually close nicer as well. (or slam really hard when angry):

Poplar is far lighter than any MDF.
It is one of the softest (besides balsa) and lightest of the hardwoods.
It excels at holding paint and crisp details.

MDF doors solid or MDF panels) are painfully heavy.

Enough that you need to make sure you use longer screws on their hinges (especially the top hinge) and three hinges are often required even on interior doors made from/with MDF.

Thanks, but still can't decide. I'm getting conflicting information from both salesmen over why I should choose their door. The poplar one says those doors are more durable than all mdf. The mdf salesman says mdf is more durable than poplar. ugh

Lupine...what did you end up choosing for doors?