Many have been asking questions on which of the two are a better camera. These two bodies have enough differences that it should be easy for majority of buyers to make decisions.
Sensor – if the rumour is correct, both cameras use the same sensor. RAW IQ of both cameras should be very similar.
EVF - Olympus E-M5 has a built-in EVF. It has the same resolution (800×600) as the optional EVF sold for GX1
but the refresh rate is twice as high which is helpful especially under low light conditions. GX1 body is smaller without the EVF mounted, but when mounted, it becomes less portable. If you use EVF often, obviously E-M5 is the way to go. If you prefer to carry light and small, you may prefer GX1. Another obvious advantage is that E-M5 will free up the accessory shoe for other purposes (such as a flash) while EVF is in use.
RAW vs JPEG – E-M5 should feature a more pleasant JPEG engine just as all Olympus bodies have in the past. Olympus knows how to produce better JPEGs than Panasonic – only few challenges this. But if you shoot RAW, then Olympus no longer has an edge. So if you shoot JPEG, then an edge goes to E-M5. If you are not into post-processing (sometimes this can be a time consuming process), a good JPEG engine is highly desirable. Some claim you can tune Panasonic JPEG engine to mimic Olympus JPEG look, but my personal experience is that this isn’t always possible.
Price/Value – GX1 is less expensive. It is still cheaper with the optional EVF but the difference is now quite small. E-M5 offers several important features GX1 doesn’t offer. So if you find features offered by E-M5 valuable, then you will find that even from pricing perspective, E-M5 will command higher value than GX1.
Style – this one is a no brainer – do you prefer the retro style or the modern style? That is up to you.
IBIS – edge goes to E-M5. GX1 simply doesn’t have any kind of IS built into the body. All Panasonic’s zoom lenses are image stabilized but none of their prime lenses are. And understandably, all Olympus lenses aren’t stabilized. If you find that you frequently get blurry shots with your non-stabilized lenses because shutter speeds are too slow, then IBIS is for you. E-M5 implemented all-new 5-axis IBIS which is supposed to be efficient enough to return 5-stops according to Olympus, which is highly doubtful. More realistically, it will probably return around 2-3 stops, which is still considered very good. Keep in mind that IBIS will stabilize your camera but not your subject. IBIS will NOT stop motion blur!
Wireless TTL Flash – this one is simple, E-M5 offers this while GX1 does not. None of the Panasonic bodies offer this.
Built-in Flash – GX1 features a built-in flash, boasting GN of 7.6/m at ISO160 . E-M5 comes with an accessory pop-up flash (FL-LM2), which boasts GN of 10/m at ISO200. Both are not a replacement for a full sized flash but useful for fill-flash and a trigger. E-M5 users will have to carry the flash around which can be a pain.
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One important addition for the all-new 5-axis IBIS: it stabilizes during video as well! So you will have stabilization during still and video! This is very important keeping in mind when shootin stills and video with prime lenses! All the video will be stabilized! This cannot be achieved with panasonic bodies as the primes are unstabilized