Since the A7(R) cameras were announced, I've often wondered where they sit in relation to the RX1. Are they simply better & more versatile? Does the RX1 offer something the A7(R) cannot provide? I've been asked this question a few times, and following on from my RX1 review, I'd like to expand my thoughts on the matter, now that I've had some time with the A7R.
One lens vs (almost) all lenses
The first question one must address when considering the RX1, is -- do I want a 35mm lens? If the answer is no, then the rest of this comparison can be summed up rather swiftly. The RX1 only has one lens, and any extenders or wideners will only hamper image quality. It's possible to crop the 24MP files or course, but if you simply do not like the 35mm focal length, then avoid this camera. Fortunately, I was smitten with the 35mm focal length after using the excellent Sony E mount Zeiss 24mm f/1,8 Sonnar T lens on my NEX-7. Due to the "crop factor" of APS-C, this lens has the full-frame field of view equivalent of 35mm (which is calculated by crop factor x focal length, i.e. 1.5 x 24 = 35). Though f/1,8 is technically faster, and allows more light, the huge sensor of the RX1 overrides any advantage of f/1,8. The RX1 Zeiss 35/2 lens is mated to a 24MP full frame sensor, which almost gives you the feeling of night vision. The RX1 lens is almost perfect. I think that there are few 35mm lenses which could "beat it" when mounted onto an A7(R).
The A7(R) cameras also have full frame sensors. They also have the ability to mount almost any lens made in existence, due to the short flange distance of the Sony E mount. There's an FE 35/2,8 Zeiss lens, which doesn't let in as much light due to the narrower aperture, but turns the A7(R) into almost the same camera in many respects. The A7(R) has more dials, more knobs, and is therefore slightly more customisable than the RX1. In practice, I see very little difference in usage, however. The RX1 lens has aperture control on the lens itself, whereas the A7R will control aperture on FE lenses using a dial.
There are comparisons of the 35/2,8 to the RX1's 35/2 all over the net, and I'm not sure there's much in it. I'm less bothered my lens charts, and ultimately judge lenses on real-world performance - both are excellent in this regard. It's also worth noting that the FE 35/2,8 on the A7R gives around the same exposure (all things being equal) as the NEX-7 and 24/1,8... with the former in a much smaller overall package.
There's no way your RX1 can do this
The A7(R) series are full system cameras. Though lenses are somewhat lacking (it is early days), you can't mount other glass to the RX1. The vertical grip offers an additional battery cubby and the ability to shoot vertically in complete comfort. The RX1 can be a little uncomfortable to shoot in portrait, but it's not a deal breaker. Either way, you can't get anything remotely looking like the above, from an RX1 setup.
That's also a good thing. A single lens, single camera setup will really focus your shooting. There's no "which lens for this scene" conundrum. You frame in your mind, frame in the RX1, and (silently) snap. You begin to see the world in a 35mm frame. This is a good thing. It will focus your photographic view, and artistic mind. Many hoped for a 50mm variant of the RX1. That would definitely be interesting, but I think 35mm was the correct choice for an all-in-one.
With the RX1, one has the choice of the 'ump
Got the hump
Oh, that hump. I've read numerous reports in gear acquisition support groups (camera internet forums), lambasting the hump of the A7(R), saying that the rangefinder style of the NEX-7 is far better. At first, I was inclined to agree. The NEX-7 looks more futuristic, but there are issues. With the NEX-7, it's not so easy to take portrait photos, as the EVF is in the bottom left. Furthermore, if you want to use your left eye for shooting, it may be a little uncomfortable. The A7(R) has a better EVF than the NEX cameras, but that is to be expected. I find little-to-no difference between the A7(R) and RX1 EVFs. They could be the same unit. It's nice to tilt the RX1 EVF, but then the screen of the A7(R) tilts... which matters more to you?
Weathering the storm
The A7(R) cameras are weather-sealed, the RX1 is not. The A7R won't be shooting underwater videos any time soon, but if the weather forecast is sub-optimal, I know I'll go for the A7R every time. The FE lenses are also weather-sealed, which is impressive, given their size. Non-FE lenses are not weather-sealed, though due to tight tolerances, many Leica-M mount lenses are supposedly rather resiliant to dust and moisture, though I will not be testing this out.
Stealth
This is a biggy. The A7(R) cameras are loud. They sound like cameras. No hiding it. The RX1 is silent. Well, not completely, but if you were to sniff and press the shutter, no-one would notice the sound of the camera. One must choose between the 36MP A7R and its loud shutter, the slightly quieter (due to Electronic First Curtain) 24MP A7, or the near-silent 24MP RX1. The reason I ponied up for the A7R was because it's the camera most different to the RX1.
Wrap-up
Which is the right camera for you? Well, it comes down to what you want. I feel that the A7R may have slightly better autofocus than the RX1, but I wouldn't put money on either beating something like Fuji's APS-C X-T1. The Fuji camera looks very nice indeed, but I have to draw a line somewhere! For now, that line circles both the RX1 and A7R. They're different enough that they can live side by side - with the RX1 used for snaps and for when I want high quality in the smallest package, and the A7R on dedicated photography outings.
Perhaps strangely, my recommendation to many, would be to get the RX1. Hear me out. I think it has improved my photography, and hopefully continues to do so. I think this is because of the sinlge-lens approach. Primes are a good first step, but sticking to just one for a considerable length of time, is bound to improve one's images. I could absolutely live with just that camera - I think the RX1 will go down in history as one of the finest cameras ever made.
The A7R offers something else entirely; 36MP with no AA filter, with the ability to mount onto it almost any lens in existence (though purchasing said lenses is another challenge). It's a system camera, waiting on a lens lineup, but offers astounding image quality in a near-pocketable package. If you have lots of legacy glass, including Leica-M mount lenses, then it's worth a look. The files are breathtaking - it's like medium format sometimes.
Good luck with your choice(s) - and remember that these lovely gadgets are just tools. The real vision comes from you.