The website quesabesde.com did the first comparison.
Here what I could understand from the translation:
The image quality is nearly par, with the SX1 a bit worse in the purple fringing.
"Without major differences
Leaving aside certain details (the same zoom, their screens and uneven connectivity HDMI making the SX1 IS), the great hope is that the CMOS latter is able at last to address the traditional limitations of those who suffer compact models with high sensitivities
However, having these first samples before a still life with fairly low lighting, obvious that there are no major differences between the two models when it comes to noise control."
The SX1 seems to have a better auto white balance.
"Anyway, apart from the various responses in terms of color, white balance, with artificial lights is more than that of the IS-SX1 and contrast, we have to look very closely at the results of these two compact to find traits that mark a boundary between them."
The SX10 seems a bit sharper to me in the lower ISO.
"From ISO 200, noise is already visible in the images taken by both models, while not becoming a serious problem up to 800 ISO. At these levels, in fact, it IS the SX10 which suffers less from the ravages of processing and noise reduction system."
The SX10 looses more detail in high ISO due to smearing.
"It is with the lowest sensitivities when it imposed the SX1 IS slightly, proving to be capable of offering little more detail and sharpness in the images."
The display from the SX1 seems to be clearer for the testers.
"The size of the brand since the early differences between the two. Thus, the SX10 IS presents a display of 2.5 inches and 230,000 points, while the SX1 IS raises the diagonal up 2.8 inches and puts on a 16:9 widescreen format, very useful for recording videos (though not so much for catching photo).
But the similar specifications and the same resolution, the truth is that the quality displayed by the LCD of the SX1 IS in the preview and review of the images is quite superior."
The video mode is what sets these 2 cameras apart, If you do not need the HD video, the SX10 is your camera. If you wants it the precious Full HD then the SX1 is your choice. Be aware of the limitations:
"While overall performance is pretty decent, the main problem is that it IS the SX1, to be equipped with a shutter-type "rolling shutter" in the fast movements of panoramic shots or more volatile straight lines appear visibly leaning. An effect that also suffer from the Nikon D90 or some video cameras equipped with CMOS sensor.
In this sense, the above-mentioned Casio EX-F1 shows superior performance in the sequences in high definition, we assume that thanks to the increased speed of response and its CMOS processor."
Also if you want to watch or convert Full HD on your computer, it better be a fast one.
Here are 2 screen captures to compare actual footage
(click image for original size)
compare via picture in picture