Number of USB Ports on Controller

I am trying to select a controller for a RT system roll out.  We will be using USB cDAQ chassis for higher channel densities (30-120 channels).  My program does not need much computer power, so processor/memory wise a lower budget controller will work.  I may need more than two USB cDAQ chassis however.  Is a 4-6 port PXI controller USB bus going to be any better than a 2 port with an external hub for my application?

Should I be considering USB 3.0 for future cDAQ USB chassis?  Will I see a performance improvement on current cDAQ USB chassis with USB 3.0 over USB 2.0?

Thank you.