What is a better way to setup mongo?

I have a set of documents that are the same but could be sorted into two distinct groups based on usage.

One of these groups, we'll call them "current", has a low volume of documents being queried a lot.

The other group, we'll call them "backlog", is a huge volume of documents queried much less often.

My thought was that not mixing these these two types of the same document would allow me to query the very active "current" docs without needing to sift through the huge amount of "backlog" every time.

Should these be in two different collections or the same "cars" collection?

mongo.collection('cars', function(err, cars){
    cars.find({type:'new', color:'blue'}).toArray(function(err, newBlueCars) {
        //do something with newBlueCars
    });
});

mongo.collection('cars', function(err, cars){
    cars.find({type:'used', color:'blue'}).toArray(function(err, usedBlueCars) {
        //do something with usedBlueCars
    });
});

OR

mongo.collection('cars.current', function(err, cars){
    cars.find({color:'blue'}).toArray(function(err, currentBlueCars) {
        //do something with newBlueCars
    });
});

mongo.collection('cars.backlog', function(err, cars){
    cars.find({color:'blue'}).toArray(function(err, backlogBlueCars) {
        //do something with usedBlueCars
    });
});