Always a tough question since so much depends on shooting style and personal tolerance for IQ and camera size. I have both the X10 (recently bought) and the X100.
I will preface this by recommending you read the DP Review Fujifilm forum and check out the White Dot (or disc) Syndrome. This is real and it has the potential to ruin a shot. At this time I would recommend waiting until Fuji has addressed this issue. I lived with the 5DII "black" dot and WDS is far worse. A second, minor point, is Lightroom does not have RAW support for the X10 yet.
Assuming Fuji fixes the WDS, or your style of photography does not have much WDS then the X10 is capable of outstanding photographs. Fuji has pushed the limits of what a small sensor can do and coupled it with a very well matched lens ranging from f2.0-2.8 even at the telephoto end. The S95, for example, goes to f5.6 at the telephoto end and so is noticably soft as a result. Results in good light are really very good and results in low light are much better than other compact cameras.
The X100 is also an outstanding camera. A little quirky to use (as is the X10) but gives results comparable to the 5DII at comparable ISO. It lacks versitilty due to the fixed lens. However, the high ISO results from the X100 surpass those of the X10 without questions. Physics is physics and a small sensor simply doesn't collect the same number of photons as a large sensor.
I have the 5DII and when it matters it is the camera I take. On the last two major outings I took the X100 as well for use in the evening or when I didn't want to pack all the gear. For people shots in the evening, at the restaurant, or on the street the X100 is nearly ideal as is the 35mm equivilent focal length. It is so quiet!.
The less talked about issue is sensor size and control over DOF. One the advantages of the full-frame sensor is control over depth of field. The smaller the sensor the harder it is to do. This is another reason I take the 5DII. I often feel I don't get good bokeh or control over DOF even with APS-C size sensors. The X10, has, of course, very limited in control over DOF.
So comparing the X100 to the X10 is a bit of a choice between a better ISO performance trade off with less focal length versitility. An expensive alternative is the M9 which gives a bit more versatility with more weight. When using LR noise on the M9 is acceptable even in fairly low light.
For me the X10 is a camera to take with me everywhere, with a zoom when I don't want to take any gear. Because of the zoom it is more versitile than the X100 and does OK in low light. It is that "camera you have with you" so it becomes the best camera. If/when Fuji fixes the WDS that is what I will use it for. Due to the size and difficulty in using it, the X10 is not suitable for the less nerdy half of my personal partnership for whom the iPhone camera is mostly used because it has a single button.
In the end it really comes down to how much equipment you are willing to carry for any particular outing. The X10 is the best of the small (assuming no WDS), the 5DII (with fast L lenses) is the best of the large (IMHO). The X100, K5, GH2 etc, are all in between. I have searched for the ideal solution and there is not. So in the end nothing beats the 5DII for IQ, low light and versitility so it always goes when it matters. After that, it is how much I am willing to carry.
Currently I carry the Pentax K5 everywhere because of the size and auto-iso modes. It does a great job but it still isn't a 5DII and I don't like the Sony senor style of colour and the dead shadows due to pre-RAW noise reduction.
