Lenses should not be a problem. There are plenty of 3rd party lenses from Sigma, Tamron and Tokina which supports Sony mount. You can also use Minolta lenses on Sony bodies and best of all, enjoy stabilisation on all these lenses as the stabilisation is in the body.
The main advantage canon 550D has over sony a550 is the video mode. Image quality of both camera will be similar. Sony's advantage are HDR, build in image stabiliser (ALL lens becomes stabilised),the tiltable lcd screen to allow over the head or ground level picture composition, and sony has the fastest af live view.
Don worry about lenses,sony alpha can use the hundreds of minolta lenses available. Minolta lenses are cheaper than canon, nikon and sony lenses usually.
So decide between video vs functions.
ADD: if u r into sports photography or wanna take F1, u r gonna be addicted to sony a550's 7fps shooting speed. It sounds like a machine gun lol.
Lens wise , if you count the model number , Canon definitely has wider ranges.
Practically speaking , one dont really need so much lens ?
For the Sony/KM mount , the general ranges are well covered from ultra wide to zooms. Normally pple qoute sony lens are pricier , that is true if you simply look at RRP pricings from Sony website or galleries. And also , they tend to qoute the Carl Zeis lens.
I mean , AF Carl Zeis lens leh . Cant expect them to be cheap rite ? haha. Anyway , if you look at the pristine 24-70 2.8 versions from Nikon , Canon and Sony. They all come close in terms of pricing ( Correct me if I am wrong, as 1.5 year back all 3 brands sell ard 2.5k )
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.
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
* 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
* 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