Watch both of my how to build a spray booth videos. In the first one I just had a negative pressure fan, like most professional booths. The issue is, that they are properly sealed, rigid units, so it works well. In my second video, I get another identical fan and have one pushing in through a filter and one sucking out. The fans work better at pushing than pulling, which makes the booth positive pressure. There are a heap of benefits to positive pressure in this situation. For starters, the sides push out which gives you heaps more space to work. The biggest benefit though, is that it is always pushing any dust out. As it is a tent, it is not completely sealed, and if it is negative, dust will get in where ever it can.
My 911's paint is a little dry in a couple of spots, where my painting skills fell down, but there is basically no dust in it at all. It is better than half of the professional booths I have worked in.