Having owned both cameras it's pretty easy for me to answer which I prefer shooting with. But what I prefer may not be relevant to your shooting style or habits, but I'm give you some brief comments about what I like and don't like about each.
- Great IQ
- Video quality and OIS is first rate. Video is sharper with less artifacts, and more effective stabilization than the other 1080p60 AVCHD camera I've owned, the LX7. And video seems to have significantly better DR (the LX7 blows highlights much more easily than the RX100)
- Small size. It's not jeans pocket small, but close to it.
- Decent control like the lens ring that allow you to change various setting depending on mode
- Very good DR
- Good high ISO ability for it's class, similar to my Nikon V1 and slightly worse than the V2 (the RX100 scored 390 ISO on DxOMark Low-Light ISO (sports) score to top all P&Ss
- No VF (Shooing with the LCD, mobile phone style is about the least stable way to shoot and composing and focussing in bright light is near impossible. That said, the LCD is higher res than the X10/X20 but for whatever reason Sony LCDs just attract fingerprints like magnets. Obviously an LCD protector or LCD shade is a good idea).
- Lens soft at f1.8
- Poor macro mode (the X10 with it's Super Macro mode allows extremely close focus with the front element of the lens practically touching the subject. To me, P&Ss with their deep DOF are superb macro tools, so this was a huge disappointment for the me with the RX100
- Slow f4.9 lens at full telephoto - This is a biggie for me, as a lens at portrait length that doesn't allow you to through the background OOF is kind of limiting. And having that f4.9 max aperture pretty much kills the DOF control advantage of the larger 1" sensor. Since f1.8 at the wide end is soft, and the long end has a slow max aperture, I felt a bit cheated.
- No hot-shoe (this may matter for some, not so much for others)
- No grip whatsoever, but the camera is so small, you can simply rest it in your palm and cradle the lens with your left hand. OR you can likely add a third-party grip
- Electronic shutter with recorded shutter sound - I know that electronic shutter can be totally silent, and can provide fast burst shooting, but I just hate the cheesy fake shutter. It just makes me think of a cheap Coolpix. Not a fan at all. On my Nikon V1 I almost ALWAYS have it set to Mechanical shutter.
- No filter thread
- Expensive. Sony cameras rarely, if ever, go on sale. The RX100 is the same $650 it was when released.
- Great handling with decent grip
- More external controls, hard buttons and dials like EV compensation, Q button, etc. As someone who shoots A frequently, having an EV comp dial under your right hand is great
- VF - This is huge. There are very few high end compacts with OVFs and now with the updated VF which includes shooting data and AF points, the VF will be even better. The VF just makes the X10/X20 shoot much more like a proper camera and not a cheesy P&S
- Hot Shoe, extremely useful, good to have
- Much prefer Fuji colors
- The fast f2-f2.8 manual zoom is significantly faster to change focal lengths quickly to follow fast action or for street photography where you need to react quickly. Love the manual zoom.
- OOC JPEGs are the best I've ever seen from any P&S, possibly any camera. There is almost no need to shoot RAW (I said almost). I'm a RAW shooter, but there are times when I shoot RAW + JPEG and the JPEGs are so good, I don't even bother converting the RAWs.
- X20 Stuff - PDAF should give the X20 the best AF of any P&S. As a Nikon V1 user, unless you've shot with the Nikon 1s, you cannot imagine just how solid the AF Single and AF Tracking are. If the X20 even approaches the Nikon 1 cameras it's going to be an awesome camera. The X10 has quick, accurate AF. The X20 should be miles better with the addition of PDAF. Updated VF, which I mentioned above. The new X-Trans sensor with no OLPF (no AA-Filter) should increase resolution considerably (Fuji is of course claiming it does). Having used the X-Pro1 which uses the same X-Trans architecture, I can tell you the files are detailed and sharp. New movie specs with 1080p60 at 36 mbps. I don't remember what the RX100 bit rate is, but I'm extremely excited about this new update. The X10's video was not anything special, but the X20 has a chance to change all that. By the way, the RX100 also shoots 1080p60, and the X20 video is going to have really be good to match the RX100's video quality. We'll see. Lastly, focus peaking something I had on my K30 and it's is truly a joy to use a camera with focus peaking.
- X10 has all the modern gizmos like sweep panorama mode. This is actually quite useful and does a good job. I'm pretty sure the RX100 has sweep panorama as well.
- Film modes - As a Velvia and Provia shooter, I like having these modes.
- Fuji Colors which with the new X-Trans sensor should be even better.
- Good high ISO ability for it's class, only about 1/3 to 1/2 EV worse than my Nikon V1 (the X10 scored 245 ISO on DxOMark Low-Light ISO (sports) score test to top ALL P&S but the RX100. Better high ISO images than ALL the 1/1.7" sensor cameras like the G15, P7700, LX7, etc.
- Bigger than the RX100. The X10/X20 are almost as large as a Panasonic GX1 with pancake lens. For cameras that I want to use seriously or semi-seriously, I prefer cameras, but some don't. The X10/X20 is not pocketable as far as pants. A coat pocket yes. I always use camera bags or pouches so this is not a big deal for me, for others it might be.
- The X10 VF had no data, but fortunately the X20 has updated the VF with shooting data and AF points. Nice. Also some zoom setting, you can see part of the lens barell in the corner of the VF.
- RAW processing - Software vendors have been slow to fully support X-Trans so we'll have to wait and see how this effects the X20
- The original X10 sensor has issues with blooming (white orbs). This was corrected on the X10, and should be only a distant memory with the X20. This problem was much of an issue at all for the kinds of images I shoot.
- X10 gets about 11 EV DR in RAW which is great for a P&S, but is still about 1 EV less than the RX100. Most expect X20 to improve considerably for DR, but I've always been happy with the X10's DR, so any improvements with the X20 will be icing on the cake.
Anyway I sold my RX100 after a couple of months. I like the IQ and video was great, but I just wasn't enjoying shooting with it. It really didn't feel like an enthusiast compact like the LX7, XZ-1 or GRD III that I used before it.
I don't have my X10 anymore either, but I have pre-ordered the X20 in Silver and Black. To me, it's one of the most aesthetically beautiful cameras ever made. And it's a great shooter and one of my favorite compacts ever. Cheers, Markus