There is a noticeable difference.  Have a look at the samples on sites like dpreview and you can see for yourself.  Personally, I don't mind using my 550D, as it's good enough for microstock and alamy.  There is another option, I use autopanopro to stick photos together and I can now have whatever resolution I want.

Apart from noise (which will not be much if you stick to low ISOs), is there a significant advantage between a cropped sensor and FF? I believe lens quality will be an issue whenever you increase pixels - I know people who loved the 17-40mm with a 12MPix camera and were disappointed when they started using a 21MPix camera.

Perhaps even more of an issue in a FF because it will use more of the edges of a lens (I'm not sure about the latter, however).

The key issue for landscapes would be the width of the Pentax sensor. It is only 2/3 the size of a 35mm full-frame sensor. If you want to take wide-angle landscapes, a 24mm lens is only going to give you the same angle of coverage as a 35mm lens on a full frame camera. You'd need to go as wide as 12mm to get the coverage an 18 0r 20mm lens would give you on full frame.

Now, what sort of landscapes do you want to shoot and what focal lengths can your existing lenses offer? That's probably where your answer lies. If you are going to have to get new lenses, anyway, then the issue about sticking with Pentax ceases to matter. And you need to check how good your existing lenses are when dealing with the zero tolerance for CA etc on the micros' inspections.

With a full-frame Canon I wouldn't advise the 17-40, the 24-70 2.8L is very good, though. If you want 18mm or thereabouts on full frame, the Zeiss Distagon 18/3.5 ZE would probably be the way to go, if you have the cash. It's manual focus but that shouldn't be a problem.