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If I were going with the Olympus housing I would be looking at the port adapter from ZEN Underwater and the ZEN ports which are designed for the Olympus and Panasonic M43 lenses. This adapter is ready to go for the E-M5 housing with the same port mount and uses existing PEN housing ports covering lenses like 9-18, 7-14 and 45 macro.
Zen Underwater PA-EP08-EP Port Adaptor for Pen Domes [zen.pa.ep08.ep] - $100.00 : Reef Photo & Video!, The Underwater Photo Pros
EskilJ- I have tested the magnetic focusing system with the Inon 50/60D housing and the Olympus 50mm macro port system. It works but is not quite as easy to fine tune as most focus gears. Keep in mind that the port is designed for the Olympus 50mm 4/3 macro lens, not the Olympus M43 60mm macro lens. Athena, AeiHItBVWEB also makes a full line of ports for the Olympus systems if you can find a retailer not seen much in the US. Their line is actually larger than the Inon line.
All things being equal the Nauticam housing is by far the better choice and in the end with a few lenses and ports the cost differences will be slim.
Phil Rudin