2 Channels
Only 2 or 4 channels means that some jumpering may be required by the user to satisfy the RS232 configuration. This may be done by internal straps if available in the adapter or by soldering at the users cable.
  8 Channels
All 8 channels supported means no need for any jumpering of signals. There is no need for soldering of any new wiring in users cable. Model OP9 recreates the the full 9 pin interface. - Just plug in and begin communicating.
Optical Isolation
Prevents ground loops, signals are protected.
  Optical Isolation
Prevents ground loops, signals are protected.

Limited Flow Control Handshaking

Only 2 Channels means that Full Duplex and Flow Control handshaking is limited as both cannot be supported concurrently

  Full Flow Control Handshaking
All possible functions of interface are supported: All handshaking configurations, with Full Duplex, or Half Duplex

No Ground Isolation

Ground isolation is not always supported. RS232 ground pin 5 on the DB9 connector may be connected between input and output connectors.
The metallic ground path does not isolate electrical problems between the equipment. Such instances may involve unsafe hot AC ground, transients on ground with secondary effects, or RF interference.
  Full Ground Isolation
Fully supported, by the use of dual, separate transformers
(UL/CSA Approved AC Adapters). Two separate circuits without a common ground isolates problems such as unsafe hot AC Grounds, power transients through branches of ground circuits, and even RF transmission prevention.
Single Power Supply
This may mean that a DC-DC coupler is used inside the isolator circuit to isolate the ground. Typically, DC-DC couplers do not have high "withstand voltage" ratings comparable to the optical ratings. If no DC-DC coupler is used, then that particular isolator may not offer any ground isolation at all, as the ground line is passed through to the opposite interface side.

Dual Power Supply
Although the dual wallmount power supply system seems a little inconvenient, it has the ability to isolate the ground circuit much more effectively, with the insulative effects of a higher "withstand voltage" rating between the two wallmount power supply units.
You are seeking the best isolation available!

No Visual Assurance

There is no visual assurance that the isolator is actualy isolating the way it is envisioned. Users may not have wiring diagrams to evaluate, or understand the requirements for isolation. Users do not realize that popular isolators may not isolate the ground line.
Circuit board traces may be too close together to defeat the optics high voltage breakdown specified.
Internal defects in the isolator enclosure or even a metallic dirt particle could jeopardize the isolation and its state cannot be easily determined.
  Simple Visual Assurance
Photologic Model VA9 is two separate modules: transmitter and receiver that have no metallic connections between each other. It is visually confirmed by any observer that isolation is 100% without any possible exceptions. Inexperienced users can be assured just by viewing the device that safety and full isolation exists.