Put it this way, the A900 is good until about ISO1600, because no matter how you spin it, it's a FF camera, the size of the sensor is like 4X bigger than m4/3, it's a dinosaur, but even dinosaur FF still kick crop sensor around all day long (in fact, I think none of the APS-C sensor match it's low light capability, they come close but when you actually process the image, the A900 wins most of the time). Crop sensor high ISO the 16MP from sony still sits on top, but you need to get nikon/pentax camera, because sony uses SLT, it's only 2/3 as good in low light as the other camera. I personally would go for the 24MP crop, yeah you don't get as good of high ISO (it's close though), but you do get way more MP to play with.
the m4/3 IMO is good until about ISO 800, i stop up at iso 1600 and it's a mess, but same can be said for most APS-C camera, they tend to be just 1/2 a stop better.
landscape wise, if you hit the 36MP FF from nikon/sony, and slap on a wide angle (12mm or even 14mm), the m4/3 can't match that stuff, m4/3 does have the 7-14mm panasonic, but it's nearly 1000 dollars, you can pickup a sigma 12-24 for a FF at roughly 1/2 (I once saw 1 that was 350 at a retail store and someone picked it up before i made up my mind).
But the A900 is bulky, it's a brick, i hate to haul that thing around.