It depends. There's a billion types of foam ones. The last set I got were really foam fluffy and I could really use them right so I went to town with with electrical tape. And their now almost as hard and as heavy as my real ones.
I'd say get the ball bearing ones. Some of the issues I've had with padded nunchaku were the padded part works it way down as you get faster, stronger with it. and sometimes it'll pinch you. You can fix these issues with electrical tape.
I learned by one of my masters just tossed me one , oneday. He showed me stuff here and there. He told me to do whatever thing with each arm. Then one day he tossed me another one. Then I just kind of went on my own from there, progressing, challenging myself etc. At the same time I don't treat it like a fuckin spirit baton like some of the people on youtube do. But I will do stuff like run and use them, use them blind folded.
I know some of the movements for technical attacks, blocking, chokes from my master, books, dvd etc. But then I thought "when the hell am I actually going to have these around in a fight?" You're more likely to have a stick or knife.