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.
For its time, the A900 was the full-frame camera with the most MP/$ and had substantially better DR than anything else under $8k. But today, a good 24MP APS-C sensor (like the one in the D7100 or K-3) will give you equally good low-light performance and better dynamic range, so unless there are some special lenses that you have in mind, or you really like the ergonomics of a large SLR-style camera, I'm not sure there's much of a reason to go for the A900 (or its younger brother, the A850).
^^ What he just said. The only thing that you will loose/gain depending on your perspective is lose shallow DOF/gain more DOF at at the same exposure value. Colour depth, dynamic range, tonal range, handling of roll offs to highlights and blacks, even noise control - these days are all metrics of the underlying silicon technology itself, not a direct result of the physical sensor size (in crude terms it explains why your iPhone 1 doesn't take as good pics as say your iPhone 5 with a similar sensor size does today ~ I'm generalising but you get the picture ). Indirectly it is 'easier' to suppress noise with a larger pixel pitch on a larger sensor with good in-body processing, however this in itself doesn't tell the entire story. Newer silicon designs that have better light sensitivity characteristics, more efficient light absorbing pixesl are more important to noise control than purely the physical size of the pixel in and of itself. In those terms, a modern m43 , Sony based APS-C sensor or small format camera (A7r, A7, D800 etc..) will have significantly better dynamic range, tonal range, color reproduction, noise control to an A900 which was cutting edge when announced in 2008. If you already have a bunch of lenses for your A900, maybe you should also consider the A99 ? I personally think it is the best non-mirrorless DSLR/DSLT available right now, particularly on account of its in-body stabilization. If you have some older Minolta lenses they would all be stabilised and you would get significantly better noise control, dynamic range etc... to your A900. If you want to break from the A900 and go mirrorless, another option would be to pick up an A7/A7r and the Sony A-E mount adapter and use your existing lenses. The A900 will be quicker than an A7R in terms of use. If you want the most versatile mirror-less camera system available today IMHO, then you would be well served by the EM1 and the excellent collection of native lenses available. http://www.stevehuffphoto.com/2013/...ew-the-most-versatile-mirrorless-camera-ever/
For IQ I would pick the A900 over any of the current crop camera out there, even the D7100, there are lots of people out there that will get the D700 over the D7100... There's more than just the numbers. it may be just my imagination but my FF sensor seems to produce better IQ at lower ISO than a crop, at the same time it feels easier to work with the RAW, they seems to take more abuse without noises popping up (yes I know it makes no sense, maybe it's just a placebo effect, but other people seems to have experience the same thing). Further more, the A900 is compatible with all the old minolta lens, and have IBIS, you buy a D7100 which is close to the A900 price and then having to buy expensive OS lens, you'll end up spending more in the long run, in fact, I like the built quality of the A900 and it's gigantic VF over any of the crop APSC i saw. the A99 on the other hand is a beast, best semi pro body out there IMO, with the 3 hindge screen, and the IBIS.
I currently own the a900 and yes it's a brick. Never had low light issues and I use old minolta glass which I find extremely cheap on keh.com I also have the a77, and I honestly do not like it all when put side to side to my a900. Again that's just a personal opinion. I am losing the a77 and going m43 for the lugging around issues. A lot of these m43 cameras do very well at low light and I think will give the a900 a run for it's money depending on lens quality of course. There's a lot to consider when comparing cameras because lens is a huge factor and will really hurt or help your camera depending on what you plan on using for.
