A question for those who have owned both: In practical shooting terms, how much better and faster is the E-M1 over the E-M5? I'm having a hard time getting a handle on the practical differences. I'm most interested in the viewfinder lag, focusing (especially in lower light), and general handling.
Another consideration is standard 4/3 lenses. I still have my original 14-54 f/2.8-3.5, and the original 40-150 f/3.5-4.5. Neither have the later optimization for contrast autofocus. How well would these lenses work on an E-M1?
Saying "which is better?" has so many assumptions in it that it's difficult to say anything with credibility without driving into the details. These are two rather different cameras, although they're from the same family and share a lot of design points.
For me, one huge difference is that the E-M5 controls are too cramped and the camera feels too small. I tried one when they came out and it didn't appeal to me because of that. Never got any further with it.
The E-M1 model is subtly a bit larger, with larger buttons and controls, a solid grip, lots more customization capability, improved image stabilization, and a higher performance viewfinder.
There's no great difference in potential resolution or dynamic range of the imaging system, the two cameras are very close on those scores. The E-M1, however, contains lens correction information for all the Olympus FT lenses in its JPEG rendering engine.
The E-M1 sensor has on-chip PDAF sensors which allow FourThirds SLR lenses to focus about as well as they did with my E-5, on average, where on the E-M5 they are very slow and laggy to use with AF. (The E-M1 and E-5 focus behavior is different in many ways, so it's not quite true that all FT lenses focus as well as they do on the E-5 in all situations, but on average—the way I use a camera—they're very comparable.) This model was specifically designed as the SLR to EVF model line bridge camera at the pro level in Olympus' range, so it works darn well with Olympus professional grade HG and SHG lenses.
A lot of people who have no FT lenses and prefer the more compact camera think the E-M5 is 'better' ... People like me who have a small but very nice collection of FT lenses, and want the more spacious control layout, and customizability, etc, feel the E-M1 is a vast improvement.
In the end, they're both such good cameras I urge you to go to a store that has both with a few of your lenses and try them out.
G