Externally, the Olympus OM-D E-M5 and the rival Panasonic Lumix DMC-GH3 couldn’t look any different though they do share a number of similarities in their core specification, including sensor, EVF and articulated rear display panel. Although the video capabilities are quite impressive, the OM-D is principally designed as a stills camera, whereas you could say the reverse is true of the GH3. This likely accounts for the stronger AA filter and the slightly lower DxOMark lens scores when compared with the E-M5.
Both makers have however adopted dissimilar and conflicting stabilization systems, making cross-system lens choices more tricky. Somewhat ironically, the E-M5’s integral 5-axis stabilized sensor system is generally considered to be superior, especially for handheld video, to the in-lens based optical image stabilization technology currently favored by Panasonic (not to mention, Nikon, Canon et al). The preference isn’t based solely on results between the two, but also on the fact the Olympus system is more flexible, allowing the, not in-considerable, benefit of image stabilization with any lens fitted regardless of its focal length, type or construction (or use of such a lens with the required adaptor). That said, with the announcement of the new Lumix DMC-GX7, complete with in-body image stabilization, there’s a strong indication Panasonic may follow Olympus with all their future models.
Apart from that, and image quality, one other significant concern for potential purchasers must be autofocus. Both cameras adopt contrast detection AF, which is relatively fast and efficient in both models but it is also somewhat dependent on the focus motor and mechanism adopted by each lens. Fortunately, both Olympus and Panasonic have gone to considerable lengths to make the AF drive system suitable for both movie capture and stills. In the past, providing the lens focused accurately and quickly that was really all that mattered for stills, and little or no additional consideration was given to how smooth or quiet the system was in the process. Although ultrasonic type AF systems are for the most part quiet, they were not originally designed or intended for video, therefore, Panasonic, Olympus and Sigma (and others) are adopting linear driven AF systems on their latest models that allow for these small, rapid, continuous and smooth adjustments. Look out specifically for the MSC designation for Olympus lenses.