Big girl bed mattress Qs

DD's been in her converted crib/toddler bed since the summer, but her twin bed is arriving later this month. What kind of mattress is best? I'm thinking firm since crib mattresses are firm and so she's used to that.

Also, her bed frame can be used with or without a box spring. Do you think it's a good idea to start without since she'll be a little lower to the ground? (We'll be using a rail regardless.)

TIA!

Re: Big girl bed mattress Qs

We got a "low profile" box spring for one of our daughters. The other has a platform bed without a box spring. I like keeping them a bit lower.