I definitely prefer native speaker because they know how to sound the language more natural .  

If I take who study it , they may be correct in grammer but may be they are not able to talk like native speaker . 

Native speakers. However, sometimes it can better to ask for a help a non-native speaker. In my case, I would be happy to meet a Slovak who can speak American English almost like a native speaker. I would ask him/her for a piece of advice how he/she was able to learn all of those difficult American sounds that we do not use in Slovak language. He/she might be able to explain that better than a native speaker who does not know anything about Slovak language, ... how we use mouth when producing our sounds.

I just thought of a circumstance in which I would practice with other people at my own level -- If we formed a study group and had at least one native speaker in it as well.  If there is somebody to check us, I think we can benefit from each other's insights and even mistakes.  It also might encourage people to speak more freely, because they might be less fearful of making mistakes among their peers.

Which is why formal classes are still useful, as long as they don't get stuck in oldfashioned, boring teaching methods.  For me they usually progress too slowly though, so I prefer self-study.  But I would really like it if I could learn some of the local First Nation's languages in classes.  Study materials for them are very hard to find.

I practice grammar and vocabulary with fluent non-native speakers but I prefer native speakers for pronounciation, intonation, colloquialisms and the general feeling of the language.

Native speakers have more ways to express themselves in my target language, they use more original structures and they can also tell me when a phrase I learned from an old film or book is outdated.

It depends on the day's learning goal: grammar, words or expressing myself naturally?

Woth native speakers, other students also make mistakes like you and you may end up picking up their mistakes.