I "upgraded" from a Minolta 7D to a Canon 30D. And well, what a disappointment!
The focus is faster, and the lens I got for the canon are USM, so it's a pleasure to focus, but talking about Image Quality, really, there is almost no improvement if any... the definition is a little higher, but that's all... 6 to 8Mp... I wanted an improvement in low light performance, I saw some test that said the 30D was so good at high ISO, but that's really bull****. The test are made in JPEG, but the RAW is aweful! my minolta was doing better at ISO 800 than the 30D at ISO 400...
Considering the alphas are the successor of the Minolta 5D and 7D, really, if they are cheaper in your country, then go for the sony!
I used a alpha 100 and really liked it. It felt like a lower version than the 7D, the 7D is semi pro when the alpha 100 200 300 are entry level, the 700 is the semi pro version, but the difference in image quality is not very hight, it's more about the built and options

Stil I would rather go for the 700 if I had to go for a sony, but I'm more considering about selling my 7D (I still need better in low light) and 30D to get either a Pentax K20D or a Canon 5D...
The 5D is far more expensive, and older, and as I'm a little disapointed with Canons... But well, I have 2 primes I could use on the 5D...
The K20D is not as good as the 5D in low light, but seems good enough for me, better than my 30D and 7D: can shoot at ISO 800 without much noise, 1600 and 3200 with noise, but "manageable" (like what I got in ISO 800 with the 30D and 7D, it's noisy, but ok that's manageable).
With the 30D in ISO 1600, the noise is unaceptable. And I'm using the RAW, I just can't do anything because the definition is badly affected.
This is taken with the 30D:
http://www.thirrouard.com/divers/zenphoto/maclan17/
some photography were taken in ISO1600, it's pretty easy to see, it's dark and when you watch even in modest size you can't see the details on the face...
it's REALLY grainy and I had to put in black and white because the colors were aweful and noise ninja wouldn't do the trick...
So yeah, don't listen to people who talk about Canon and Nikon like gods. They aren't.
They just are brand, and Pentax and Sony are very good brands too.
The only "bad" brand for now is Olympus, because they use very tiny sensor, and right now it's just a stupid strategy... less bokeh, less sensitivity... when we see everybody else go for FF...
If you're deciding on a Canon, look at a 30D You can get them for $799USD new at
bhphotovideo.com and you can find them for probably $600-$700 used and in very good condition.
It's superior to the XTI. I'm not sure about the Sony specs but the 30D is 8mp, has spot metering, 5 fps, faster AF (than the xti), a top LCD, scroll wheel for much more intuitive controls, and other things that make it a very, very good camera for a good price. It's the last gen of the XXD series as the 40D just came out, but it's by no means an underperformer.
Sony just announced that they developed a FF sensor not that long ago, so if they put it in a DSLR, they'll be set for catering to a pro market. You really can't go wrong with most any system out there.