Originally we tried using a plastic crate for it's more "den-like" feel, but Sophie didn't seem to love it. She also chewed parts of it off and then started laying ON TOP of it rather than inside it. This was dangerous because it was in her x-pen making it almost possible for her to escape! So, we got rid of that and just got her a dog bed in her x-pen. We still use the plastic crate in the car when traveling.
Now we have a tiny wire crate. It's an "i crate" made for toy sized dogs. I have a little dog bed in there and when it's time to sleep we put a blanket over it (just the sides not the front). However, when we bring her to friends' houses we leave the blanket off and put her in there when we're eating, etc. This way she can see what's going on and we don't have to worry about her. She goes in there on her own and lays quietly just watching us
So, long story short, my girl prefers the wire one but it's a lot of trial and error. We have three crates, two beds, and an x-pen and she's only 5 months old!