One way to choose is to analyze the conclusion - cons from dpreview.
Seems that the Sony A550 has more JPEG related cons unless you shoot raw and spend time doing post processing.
Perhaps experience shows.
Sony A550
Conclusion - Cons
* JPEG output doesn't really do the sensor justice - shoot raw for the best results
* Default output over processed: too much contrast, saturation (reds in particular often end up overdone) and noise reduction for our taste
* Metering not reliable enough, easily fooled by unusual subjects and large skies (often over exposing)
* More auto white balance errors than we'd expect at this level
* Not enough control of noise reduction for JPEGs (high or really high)
* No Program Shift or Depth of Field Preview
* Very Limited customization options compared to competitors
* Some shadow noise visible even at base ISO in certain shooting conditions
* Despite improvements the viewfinder still not that great - hard to get your eye near enough
* Button placement is less than optimal
* In today's market, at this price, lack of video has to be mentioned
Source :
http://www.dpreview.com/reviews/sonyalphadslra550/page27.asp
Canon 550D
Conclusion - Cons
* Auto Lighting Optimizer's effect is subtle to the point of being unnoticeable
* No rear control dial (as found on higher-end Canon bodies)
* Entry-level ergonomics won't suit everyone (the EOS 50D and 7D just feel nicer)
* No in-camera raw conversion option
* Slightly soft JPEG output at default settings (but this is far from unusual)
* Chroma noise takes a fairly big chunk out of resolution at ISO 12800 (but better than the 500D)
* AF assist strobe can be annoying
Source :
http://www.dpreview.com/reviews/canoneos550d/page29.asp
Nikon D5000
Conclusion - Cons
* Control system rather dependant on multiple button presses
* Contrast detect AF so slow it's useless for most types of photography (which is often the case on DSLRs)
* No built-in AF motor restricts lens choice (though most popular lenses do work)
* Default JPEG output soft (shoot RAW for best results)
* Moderate screen resolution (and too reflective in bright light)
* Video capability rather limited
* No mass storage USB support
Source :
http://www.dpreview.com/reviews/nikond5000/page33.asp