I just received my V-Cube 5, and i already owned a Qj 5x5, i can say that the V-cube is way better than the qj, it is much sturdier and almost never pops
The Sengshou has explosive pops, therefore not a good speedcube. The Shengshou also feels very wierd. The X-Cube is a very good cube but pops if you are a rough turner. Other good option is the Maru but after a while it starts core missaligning.
As for 5x5's I haven't tried the QJ but the V-cube 5 is very good if you make it adjustable, otherwise you have to break it in for a long time.