S-Video or BNC

Well... s-video is a nicer signal than composite. Unless you're going component in and out of a beta deck, I'd say stick with s-video.

EXCEPT that from only a vhs source I don't think you'll notice ANY difference

8X8 s-video router, go here: http://www.signvideo.com/8x8audio-video-routing-switchertrue-matrix.htm

Svideo is more of a consumer connection. Although we all use it, most larger switchers will do much more than SVideo.
Svideo is nothing more than two BNCs - combined into one cute connection.
One set of the pins in an SVideo connector is the Chroma information - the other is the luminance.
You can buy breakout cables (usually called YC) to break the SVideo into both separate cables.

So, you could use a RGBHV switcher and just use two of the connections for SVideo. Or, you could use a RGB switcher and use two for svideo.
Composite is the combination of both of the separate signals in Svideo - combined into one connection.
Will you notice a difference - most people do on anything except VHS. You will notice that most VHS decks do not have an SVideo connection. Once we stepped into SuperVHS (SVHS) the technology allows the separation of the chroma and luminance - hence the Svideo connection.
We use the JVC SVHS deck - that has firewire i/o. This contains audio and video - without needing any additional analog to digital interface. You can also print back out to VHS/SVHS over firewire.

http://pro.jvc.com/prof/attributes/features.jsp?featureid=01&tree=&itempath=null&modelid=MDL101333
It's not the best editing deck - but it does work well with firewire device control.

The benefit of using an RGBHV or RGB switcher is that you might use if for switching other - higher quality sources.

no - not like that.
On an HGBHV switcher - each input channel has 5 BNCs.
On an RGB switcher - each input channel has 3 BNCs.
Example:
Input 1 - has 5 BNCs - but you are only going to use 2 of them for SVideo.
Output 1 - has 5 BNCs - but you are only going to use 2 of them for SVideo.
Thus - in this scenario, you would route your video from input one to output 1.

I have this one in my studio - used in this same scenario - http://www.kramerelectronics.com/indexes/item.asp?pic=201

FYI:an 8x8 Svideo and audio switcher: http://www.kramerelectronics.com/indexes/item.asp?pic=385

If it's any help, the pin configuration for a 4 pin mini DIN connector (commonly called S-Video when used for video applications) is as follows:

PIN 1: Shield -
PIN 2: Shield -
PIN 3: Luma +
PIN 4: Chroma +

The pinout for the larger 7 pin locking Y/C connectors used by JVC and others is:

PIN 1: Luma +
PIN 2: Luma - (shield)
PIN 3: blank
PIN 4: blank
PIN 5: Chroma +
PIN 6: Chroma - (shield)
PIN 7: blank

If you're handy with a soldering iron, you can make your own breakout cables using the above pinouts as a guide.

-DH

Or, if you're handy with a Credit Card you can purchase this:

http://www.avcable.com/Merchant2/merchant.mvc?Screen=PROD&ProductCode=SVHSB&Category_Code=S_VIDEOCABLE

Sorry David, I couldn't resist.