Treated bottom plate for basement?

Just my opinion, but I would go with the pressure treated. Alway's think of the worst case scenario. Let's suppose your basement got flooded for some unknown reason. The water then would be more easily absorbed by regular 2 x 4's. The cost in the long run would be less. Have you considered metal studs? Compare the price to pressure treated lumber. I'm almost certain it would be less. Good Luck!

Building codes require treated or other naturally rot resistant lumber to be used as the base plate whenever framing comes in contact with masonry.

That said, you can use treated yellow pine or naturally rait resistant species of lumber such as cedar, redwood, oak, mahogany, cyprus or just about any tropical wood.

Better still since this is a basement: frame using metal studs.

They are cheaper, straighter, and often easier to work with.