Switching platforms: To Sony or Nikon?

[alstauffer]

First off, I apologize if this is in the wrong place - I didn't want to put it in one of the camera manufacturer sections (Sony or Nikon) to avoid people just saying to go with whichever camera they have.

I've been into photography for quite some time, as my father was an avid film photographer, but have used point-& shoot cameras until a year or so ago when I upgraded to an Olympus E-520 DSLR.  I am by no means professional or highly skilled at all for that matter, but I enjoy taking pictures, and manage to get some shots that I and other enjoy looking at.

On to my issue, though.  Basically, I'm feeling a bit limited by the camera - both by the camera itself and the Olympus platform in general.  The lenses that are available are quality, but too pricey to be accessible to me, and since I bought the camera, Oly has put more and more focus into the micro 4/3 platform and seems to be abandoning the four-thirds sensor and traditional DSLRs, leading me to believe it's a good time to get out of this gear.  I'd also like something with video capability, and a camera that performs better in low light than the in-my-experience terrible autofocus on the E-520.

That being said, I'm looking to move to a new brand, and right now the major contenders are (depending on the used/open-box/refurbished deals I can find when it's time to buy) the Sony SLT-A33/A35/A55 or the Nikon D3100/D5100.  I mainly shoot cars, both stationary and moving (autocross, rallycross, track days), pets (a lot of indoor shooting, my main 'models' are indoor kittens), and landscapes.  Because of the motorsports I like to shoot, fast/accurate autofocus is important, as is affordability of zoom lenses.  The indoor shooting means I need acceptable low-light performance as well.  I would primarily be using the video mode for shooting cars at the track.  I'm a techie, and don't mind the idea of shooting with the fixed mirror or EVF of the Sony if the performance is there. I like the idea of constant PD autofocus during video filming, but if the Nikon is superior in pretty much every way I'd probably be willing to drop it, so my questions:

Is the Nikon usable for video use by people other than manual-focus experts? 
Given my main uses for the camera, am I considering the right replacements for my current gear, or do you recommend I look at something else?
Are the lens selection and prices comparable between the two brands?  Does the Nikon requiring in-lens stabilization drive the cost of lenses up a significant amount?(i.e. would I get a better lens for the money for the Sony, where the IS is built into the body? Or would the Sony be a significantly cheaper system due to not having to buy IS/VR lenses?)
I use my FL-36 flash quite a bit indoors, usually bouncing light off of a wall or the ceiling - is the flash affordability/availability better for the Nikon or the Sony?
Does anyone have first-hand experience with the low-light performance of the cameras I'm considering?

I have held and played with both the D3100 and the SLT-A35 and A55, and didn't have a strong preference either way.  Unfortunately, a brightly-lit electronics store and a camera with no memory card (can't try out video mode, or auto-HDR, etc.) isn't the ideal way to test a camera, so if anyone has real-world, first hand experience with either or both of these cameras, I'd really appreciate hearing your thoughts and insight!

[alstauffer]

I just found an excellent deal on the Pentax K-X system with 2 lenses. I know it's not on the 'cutting edge,' like the Sony, or as new and feature packed as the Nikons, but it has 720p, 24 fps video, which is what I would end up using for the most part anyway. I believe that the video is manual focus only, but it doesn't seem like the Nikon's video autofocus is usable anyway.

Is Pentax a solid system to buy into as far as lens availability, support, and accessory prices when compared to Nikon and Sony?

The price point of the Pentax (< $400 with both 18-55mm and 50-200mm lenses) is important to me as I'm a grad student and while I can afford the pricier cameras I listed in the first post, there are other things I could use the money for if something cheaper will meet my needs.

[stever]

a DSLR is not a good tool for shooting video of motorsports.  for practical purposes you can't zoom, and manually focusing with the longish lenses required is more than difficult (the slr-like camera that might work is the Panasonic GH-2) - none of the DSLRs will autofocus on moving subjects for video

one of the advantages of Nikon, Sony, and Canon is that used lenses are available.  since Sony has in-camera stabilization, you won't have to spend the money on a long lens that you will for Nikon or Canon

[Fine_Art]

The Sony will track while shooting. That is the main advantage of the SLT method, you don't need a fancy chip and software to predict subject position while the mirror is flipped up. The AF is always getting direct feedback from the real image. There is a 1/10th second lag to the electronic viewfinder so you have to get used to using both eyes for fast moving subjects. You have to keep the AF on the right part of the subject for it to react to what you want. If it makes sharp changes in direction you will be aware through the eye not in the viewfinder first.

[stever]

although the Sony will track while, it's use for action photography like motorsports looks a bit problematic