I have to choose between these two models. G1x with a better senzor and more MP, and SX280 HS with the new processor, 20x zoom and GPS.
I need the camera for video purpose, shooting video at 1080p, and sometimes also in low light conditions.
Which one has better video quality? Which one should I buy?
I shot great video of a deer at dusk with 20x zoom using a 280. That said, the 280 has a number of disadvantages for video, most notably the still uncorrected unreliability of the battery meter (see multiple and some very long topics in this forum) despite firmware upgrades. The battery won't last too long using it with video so you'll need spares. While it shoots good video it still isn't a true video camera. Video is produced as .mp4 files which may not be the favorite format for all video editors, though Canon's own editor is fairly reasonable. Some reviewers mention zoom noise though I suspect that happens mostly when the overall sound volume is low and the automatic sensors increase sensitivity. Even though I used the electronic wind filter I still got wind noise.
Some of your decision may depend upon details you do not provide. Is this a camera to go everywhere at all times, possibly on camping trips, or is it for more in-city events and where you don't mind lugging around equipment? Even though the 280 has image stabilization you will still want a tripod if using 20x a lot. The 280 is great in that it is still a pocket size camera and I had it permanently in a hip pouch everywhere I went for a two week vacation. I ended up taking it back to the shop though, mostly because you could not rely upon it to give reliable battery life readings, especially when shooting video (again, read other topics).
I have no experience with the other camera.
The G1-X has a CONSIDERABLY larger sensor and better ISO performance... but the zoom range is more limited. Also... part of what you pay the premium for in a G1 X is that it has a particularly large lens and sensor. It's image quality is much better than what you'd expect from a point & shoot -- but keep in mind that the zoom range is limited and you cannot change the lens... so it is what it is.
Have you considered the EOS-M? In a fairly small-ish body that camera basically has the SAME sensor that you find in nearly every Canon DSLR body -- it's the same sensor they use on the T2i, T3i, T4i, T5i, 60D, and 7D bodies... but in a much more compact camera. The lens is removeable and you can get an adapter that allows you to use regular EOS DSLR lenses.
The EOS-M actually does have a microphone-in port so you could connect a much higher quality mic -- i.e. one of the Rode VideoMic series mics which are awesome. It also has the hybrid sensor AF system similar to the 70D but I don't know if it's as good as the 70D's hybrid sensor AF (which I hear is fantastic.)
The new 70D is probably *the* camera to have, but that'll be in a whole new price range. The 70D is not yet shipping (dealers are taking pre-orders.)
I have a G1 X and can vouch for the fact that it's built like a tank -- although I really never shoot video with my cameras.. One of my "other" hobbies is astronomy. My better-half was using a G1 X with an adapter that allows us to connect it to a telescope and accidentally "dropped" the camera while he was trying to disconnect it from the telescope. It fell about 3' onto a concrete sidewalk. Immedaitely thoughts are running through my mind... is it totally trashed or will it be salvageable if we send it in for service?
Tunrs out... there's a _very_ tiny dent in the corner of the camera body (the G1 X has a metal body) where the camera hit the sidewalk and absolutely nothing else on the camera shows the slightest hint of damage. Camera works flawlessly. The dent is so small that unless I really draw your attention to it, you probably wouldn't even notice it. I must say I was REALLY impressed. I should post a photo of it -- but lets just say this completely changed my perspective on how solid this camera is. I normally think of them as delicate... I don't think that anymore.
The G1 X basically has a cut-down version of the 18MP sensor that you find on Canon Rebel EOS-M, T2i, T3i, T4i, T5i, 60D, and 7D bodies... but they've cropped it down to about 80% of the APS-C size. It's about as close as you can get to DSLR performance in a point and shoot camera. The EOS-M has a full APS-C size sensor and removeable lenses.
If you want to try the G1 X, I noticed LensRentals.com will rent you one ($39 for 4 day rental). You could try it before you decide if it'll work for you.
But if you REALLY plan to shoot primarily video, then consider an actual video camera or a camera more optimized for video such as a 70D.