A bit of a personal ramble here so if you're not in the mood for being rambled at then time to read something else...
ePHOTOzine have posted this
blog comparing the Olympus OM-D E-M5 and the new Sony A7. Side by side the bodies don't look that much different in size and if the E-M5 had been fitted with the grip or the E-M1 had been used instead the difference in size would have been even less. But, check out the photo of the A7 fitted with the Carl Zeiss 24-70mm f/4 lens. A wonderful combination, no doubt, but it graphically illustrates the size penalty that a full-frame sensor can
impose on the glass.
But that isn't an automatic win for the smaller m4/3rds sensor. For reference the 24-70mm f/4 is approx. 73mm x 94.5 mm (diameter x length) and weighs approx. 426g. The newly announced M.ZUIKO DIGITAL ED 12-40mm 1:2.8, the first of a series of "Pro" lenses Olympus has designed for m4/3rds, is 69.9mm x 84mm and 382g. The 12-40mm has a similar field of view range to the 24-70mm but, while brighter, it can't quite match the 24-70mm when it comes to shallow depth of field (by a factor of approx 1.4). That said, the 12-40mm lens is the nearest match in the m4/3rds pantheon to the 24-70mm in terms of capabilities and, no surprise, as a result it's about the same size give or take.
But that isn't an automatic win for the larger full-frame sensor on the A7 or A7r either. If the option to fit a compact walkaround zoom is wanted m4/3rds bodies have the LUMIX G X VARIO PZ 14-42mm / F3.5-5.6 ASPH available. That lens is just 61mm x 26.8mm (when powered down) and weighs approx. 95g. When fitted to the new E-M1 it should, I believe, only protrude about 7mm forward of the built in grip.
So it's horses for courses. The new Sony full-framers offer more pixels and, assuming the reviewers don't spot any "gotchas", should achieve better IQ than either the E-M5 or the E-M1 even when those cameras are fitted with equivalently massive glass. If that's the priority then Sony wins - I'll be charitable and assume that Sony gets the new FE lenses on the shelves soon. But if the pixel count and IQ, which is still not at all shabby on the E-M1, are good enough then maybe the E-M1 wins as it offers a lot more flexibility.
Of course I've only addressed potential IQ and size/weight above and a look through the specs shows up a lot more differences and, indeed, areas where conventional DSLRs still win out. Personally, I now fall into the m4/3rds is good enough camp. But I'd certainly buy an A7r as well if money were no object just for the sheer joy of ownership and for those very special landscape/portrait shots where only the best (compact system camera) is good enough!