An extra note regarding logical systems on SRX. This is only supported by highend SRX platforms (SRX1400/3400/3600/5000 series) -not branch (srx100/200/300/550/650) series and SRX4100/4200.
From what I see, logical systems on SRX will not be a good longterm solution as virtualisation of the SRX is solved with vSRX instead of logical systems in large physical devices.