poplar, as I like real wood.

If you're going to hide the wood with paint, there's no reason it has to be wood. Poplar if you were going to stain for sure, but MDF or HDF if you're going to paint.

Use whichever is least expensive/workable.

MDF edges often need more paint or even filler to disguise the texture. MDF is much more limber than wood and can be a real problem handling in 12 or 16 foot long strips alone.

Both need primer before paint.

Poplar is often called The Painter's Wood because it looks great under paint---no grain showing and is very smooth. Polpar has two distinct colors between the sap and heartwoods, which makes staining difficult to achieve even color. That, along with minimal grain, are the biggest reasons it is painted.

Maple paints just as well, but is much more expensive. Pine/oak/most other woods will have grain characteristics showing under paint.

Cutting scarf joints with MDF is problematic, since MDF chips easily. Under paint, that is not a huge problem, but filling and smoothing filler is just extra work.

One way MDF would be much less expensive to use is if you have (or access to) a table saw with addons(and at least one or better, two, other people) that can rip 8' strips from 4' by 8' sheets of stock. That equipment cannot easily be rented(if at all), and costs upwards of several hundred dollars, so unless you have a woodworking shop or friend, hiring it done negates any savings.

Wood still holds crisper edges than anything molded or extruded.

Maple 1x is too darned hard to nail; poplar is the first choice.
Flat straight wood is an untraditional look, can be coarse and severe; molded edges are not popular without reason.
Casey

I have a recently built home that didn't use wood. It is fine for ceiling trim, but every ding turns into an exposed rough ugly fibrous mess. I consider it unacceptable!

A few years ago, I replaced EVERY piece of door and window trim, baseboards, and casings with poplar which is great for painting and much cheaper than maple. I bought mine at a millworks store which was much cheaper than buying at the big box stores. They were able to custom cut my fluted casings for my open doors which was a plus, as some of my openings are deeper (not quiet standard) than others due to past remodeling/build-on issues. I did not use flat - I chose a very subtle molded design for ease of painting and cleaning.

You can save some money by going with the finger-jointed trim which is perfectly fine for painting.

One thing I will point out about poplar (or at least mine was) is that after priming, I found that I needed to lightly sand as the grain of the wood raised, giving it a "nappy" feel. I also lightly sanded between the two coats of paint for a slick, smooth finish. Woodworkers will know more about this than me, but I was told that was common for poplar.

I had two sets of work-horses to paint on and my husband had a utility trailer that I stretched the pieces across for drying. We had quite the little assembly line going that summer with what seemed like MILES of trim!

I also hung all new doors, but I went with pre-hung faux wood grain doors. I was also able to purchase these doors at the same millworks yard and they were much cheaper than Lowes and Home Depot.

Hardwood dealers just about always beat the big box stores.

The down side is that hardwood is sold by thickness (measured in quarters of an inch like 4/4, 6/4, 8/4) and is often not surfaced and is radon widths and lengths.

The grading criteria set 'clear face cuttings' and set a yield and minimum size.

Luckily poplar is still relatively inexpensive and not hard to work.

I would use Poplar.

Baseboards and casings get so much wear, MDF does not hold up to dents and dings.

For crown, MDF will "hold up", but I need another helper to actually hold it up, and handle the pieces. It is really floppy and heavy. Not my choice for high quality work.

Poplar it is!! Thanks, all!