I'm confused between two cameras. Which one to buy? Sony SLT-A55VL or Nikon D5100?

Well, sony focuses on 15 points as compared to nikon that focuses on 11 points. Also, though I have not tested but I think Nikon D5100 don't use Auto Focus but Sony do has auto focus. Sony comes with built in GPS but the image quality of Nikon is said to be better.

Both have autofocus. The A55 has an electronic viewfinder which is a touch slow and causes lousy battery life.

I think that Sony makes decent P&S cameras, but their dslrs are not so great. I would go with the D5100 all the way.

The electronic view finder o the A55VL lags a bit and makes shooting action and sports problematic.

If you really, really want GPS, you can later add the Nikon GP-1 to the D5100

Frankly only one client has ever asked me to include geo-tagging in the images EXIF data. I just included the cost of the GP-1 in their bill. That was the first and last time I used it.

Both have autofocus but the a55 has autofocus that is constantly working which makes it faster and gives you better autofocus in continuous shooting and movie mode. These cameras are really different so you need to try them both out before you commit to buying one. Go to a store and give each one a good test run. Take an SD card and take some pictures so you can compare the image quality yourself.

You also have to think about lenses. You can get some really great used Minolta lenses for peanuts if you go with Sony - try finding a 70-210 f/4 (constant aperture all the way out to 210mm) at $150 for Nikon.

The Nikon D5100 DOES have autofocus, so please take that off your list of concerns. Those are 11 autofocus points you're speaking of.

Here's a side-by-side comparison of the two cameras.
http://www.dpreview.com/...ikon_d5100

A detailed review of the Sony.
http://www.dpreview.com/...ony_slta55
And a detail review of the Nikon.
http://www.dpreview.com/...ikon_d5100

Either will give a talented photographer excellent results. But the Nikon has a wider selection of OEM and 3rd party lenses available. Also, I'm not a great fan of the Sony SLT translucent mirror system. Here's some disadvantages:
Disadvantages:
Less light reaching sensor, due to portion of the light being reflected to phase-detection autofocus array (approximately 1/3 EV or 21% in current designs).
Refresh rate limited by the time it takes the sensor to form an image; in low light this causes severe stuttering of the viewfinder image when panning (e.g. If it takes 1/4 sec for the sensor to gather enough light to form an image then the EVF updates at 4FPS).
No real-time viewfinder update at high shooting rates, the viewfinder shows the last picture taken instead of where the camera is actually pointed; mostly a concern with fast-moving subjects.[citation needed] (Note: Some DSLR's do not update the image at all, as mirror is up when shooting. However, some will give you a glimpse of what is being shot as the mirror and shutter reset for the next shot.)
The EVF has far less dynamic range than the sensor, so the EVF does not properly show what will be captured.
The bright light of the EVF in current implementations can cause eyestrain when used over long periods and can affect dark-adapted vision, causing temporary nyctalopia.
Over a period of time, the mirror surfaces can become soiled. As a result of this the mirror scatters the light rays and degrades the recorded image.
http://en.wikipedia.org/...SLT_camera

For me, the light-loss if a big factor. The one thing you might want to consider is how Sony does in Customer Satisfaction, according to JD Powers.
http://businesscenter.jdpower.com/news/pressrelease.aspx?ID=2011105

Nikon D5100 is better.
If you go for sony. Go for Sony dslr A580L.
Or you can choose. Canon EOS rebel T3i, Sony DSLR A560L.