Short List: Arca-Swiss
The Arca-Swiss quick release system is arguably the best quick release system and certainly the most universal among professional photographers. The system is simple and you can start for less than $80 and use almost any tripod head you now own. . . .
There are two parts to the Arca-Swiss system, (1) a clamp on the tripod head and (2) a mounting plate on the bottom of your camera or long lens. The jaws of the clamp grab two long ridges on the side of the mounting plate. It is quick and secure.
Arca-Swiss, Graf, Foba, Acratech, Markin, Kirk Enterprises, and Really Right Stuff all make some tripods heads that use an Arca-Swiss style clamp. Some of the heads by Linhof, Giotto, and Bogen-Manfrotto can easily be modified to take an Arca-Swiss style clamp. And finally, an Arca-Swiss clamp can be screwed on the top of almost any tripod head that has a 1/4 x 20 thread mounting stud for the camera.
I addition to the clamp, you need a mounting plate to go on the bottom of your camera and any long lenses that mount directly on the tripod.
The cheapest way to get started is to buy a $40 QRC-1 Quick Release Clamp and a $39 PZ-3 universal mounting plate (sorry, there is no photo of the PZ-3 on the Kirk site) from Kirk Enterprises. Screw the QRC-1 firmly on top of your existing tripod head and the PZ-3 on the bottom of your camera or long lens and your quick relese system is ready to use. Just drop the mounting plate between the jaws, tighten the jaws, and you are ready to shoot.
For $55 or $60, you can get the QRC-1.75 or QRC-2 and have a little wider clamp. I think it is worth the money to get the QRC-2 clamp. For your information the QRC-3 and QRC-4 are drilled for a 3/8 inch mounting stud, larger than the standard stud on top of most tripod heads..
A step up from a universal mounting plate is to get a plate that is machined specifically for your camera. These plates have a lip so your camera won’t rotate when you flip your camera over to the vertical postion. You can order these custom made plates from Kirk Enterprises and Really Right Stuff. Unless the initial cost is a serious consideration, I recommend getting a custom mounting plate that is machined for you camera body. A universal plate will hold an SLR body and a small lens in the vertical position provided the plate is screwed on tight, but the custom plates are more elegant and will handle the larger lenses.
For a step up from the custom mounting plates, get a custom made L-plate/L-bracket (I will use the word plate for both). These L-plates wrap around the bottom and left side of the camera, add minimal size and weight, and allow you to go from horizontal to vertical without flopping over the tripod head. The two best sources for L-plates are Really Right Stuff and Kirk Enterprises. The L-plates are designed so you can change batteries, connect a cable release, or access other camera connections without removing the bracket/plate.
What if you have two cameras?
If they are the same model, you can move the mounting plate back and forth. It takes me less than 60 seconds to take the bracket off of one one camera and mount it on another camera. I switch back and forth too often to want to do that, so I just got two plates. If you have two different camera models and one is a less used backup, put a universal plate on the backup and get a custom plate for your primary camera. If you use both cameras a lot, you will need two plates.
If you have long lenses that mount directly on the tripod, you will need either a universal or custom plate to go on your long lenses. Again, the best source is Kirk Enterprises or Really Right Stuff. I have custom plates on my long lenses and a universal plate on my Canon 1x-5x macro lens.
Some day you will want a ball head that has the Arca-Swiss style clamp built in. My current ball head of choice is the Kirk BH-3. I highly recommend it. I picked it because it is the most reasonably priced, highly-rated ball head that will suit my purposes. I’ve used mine for months and it is a high-quality, modest-sized, relatively light weight ball head that easily handles an SLR/DSLR with a 70-200mm f/2.8 or 100-400mm zoom lens. For a larger lens, you will need a bigger ball head.
Do I have a preference for either company? Mostly no. I’ve been to Kirk Enterprises in Angola, Indiana and visited with the owner of Really Right Stuff by phone. Both companies are really good to work with. I own products from both companies and they are very good. I do have a couple of personal product preferences, one for each brand.
My first personal preference has to do with L-plates. The Kirk Enterprises L-plate grips the strap ring on the Canon 20D for added stability. This photo shows an extra attachment that goes on the L-plate for the camera strap. Since I occasionally remove the L-plate, I want the camera strap attached to the camera, not to the L-plate. The Kirk Enterprises L-plate is inconvenient for me due to this arrangement. For this reason, I prefer the Really Right Stuff L-plate for the Canon 20D and Canon 5D.
My other personal preference has to do with “conversion clamps” (my phrase). For converting an existing tripod head by screwing an Arca-Swiss style clamp on top of the head, I prefer the QRC-1, QRC-1.75, or QRC-2 clamps from Kirk Enterprises. They are drilled to the 1/4 inch stud size typical of the top of most tripod heads. If you get a Really Right Stuff clamp, you will need to use a 1/4 to 3/8 inch bushing to screw the clamp onto your tripod head. For some specific situations, I use my Arca-Swiss style clamp on several tripod heads and don’t want to mess with a bushing. The bushing is just one more thing to fuss with and lose.
These preferences are specific to my equipment and how I do photography. For everything else I have no strong preferences between the two brands. You can’t go wrong with either company.