Gosh, just goes to show that huge numbers of megapixels are not everything...
The Fuji S3 I started with (as a DSLR) only boasts 6 mp x2 because it has two lots of pixels, one for shadows, one for highlights. Even with this I rarely used the camera on maximum pixel recording because it just wasn't necessary - it just seemed to produce unnecessarily large files.
Iso 400 was never a problem with this camera though - it seemed to be barely distinguishable from Iso 200, even in the shadows. Maybe this was a benefit of the dual-pixel sensor.
I then moved onto a S5 and it was more of the same really except for having a much larger screen and slightly better quality all-round. This camera is more intuitive to use though as it has more dedicated buttons, meaning you don't have to delve into the menu all the time.
Then it was onto a Nikon D300S, which has an even bigger screen and a different type of sensor (cmos)which doesn't suck dust onto itself all the time. This is brilliant - I haven't had to clean it once! It actually has a buzz facility to vibrate dust off but I never need to use it.
I very rarely use full quality on this camera, doing so produces enormous files and ithe image quality is brilliant even on lower settings.
My wife has a Canon 20D, which is a fine camera but with an out-dated screen-size, which is a shame. My daughter has a 5D Mark 2 which is fantastic but I can't help thinking over the top for her needs. This will produce humongous file-sizes...
She used to have a 550D, which is nice and light but the flip-side of that is that it lacks the solid, bullet-proof feel of magnesium-bodied cameras - you can't have it both ways
Nikon v Canon? I'm used to the Nikons of course but I do find them more intuitive to use. Both will produce excellent images, so it's largely a matter of what takes your fancy. My missus may well switch to Nikon though, for the sake of compatibility with my kit, so there might be a 20D body up for grabs and a mint 17-85 Canon lens which is a significant step up from the kit lens you get with a 550D.