Once it's all painted, you won't be able to tell the difference.

Only you and the builder will know it's MDF. It's definitely worth it if you're talking about lot's of linear feet of baseboard. Not that you need to worry about the installation but not only is it significantly less expensive, it doesn't split and splinter when cutting it or nailing it to the wall. Pine splits easier and becomes frustrating to work with sometimes. Also it withstands any changes in heat/humidity. As long as you're painting it anyway, MDF is fine!

If painting it will look great

It wears well, doesn't warp and takes paint beautifully. I would prefer it if I planned to paint.

BTW: The majority of my trim in my house is MDF. The only trim that isn't is my hatband and my window sills.

You guys are awesome. Thank You for all the feedback!

Solid poplar. Cuts and paints great.

MDF dents 'very' easy. I would only use it for crown where it won't get dented.

The finger joints frequently telegraph through the paint and become visible in a year or five. It probably depends somewhat on just how evenly the wood was dried prior to fabrication, but also on small variation in seasonal movement from piece to piece of wood from different trees. The former can be solved by acclimating the wood to the house and sanding it even prior to finishing, the latter can't. Something to bear in mind-- if that's going to bother you, upgrading to poplar will solve the problem.