.....
going to be painted. My question is do I make it out of maple anyway
since it will be much more durable or go with poplar to save money. Of
course you can't answer this for me but I'm curious what you think you
would do.

Maple. The cost of material is minimal next to your labor, and then maybe
it will be around for your daughter to give to her daughter. *That* would be
worth the extra $.

I have used poplar, but have gone to all maple for "white-painted" stuff.

Go with an oil (alkyd) based paint.

-Steve