uscarry45, sorry I'm a bit late to reply, but figured I'd chime in on this. I'm a bit of an SFIC system fan, as some of the regulars here probably know.
Regarding choice of keyway, it sort of depends on what you're after. First, I'll say that any SFIC lock/keyway is going to be miles better than anything you could find in any big box store ever. Especially if you stick the the actual Best name brand, their cores are machined to really tight tolerances.
For my personal stuff and the locks I service on behalf of a few clients, I tend to go with one of a handful of the W* keyways. I like them because the thick key is quite durable, the heavily paracentric keyway is difficult to get a pick in, and even though they're not restricted stuff for them is not terribly common (meaning there would be little overlap with other key systems in your area). The keys are also quite difficult to cut even for an experienced person. Since the keys stop on both the tip and the shoulder your spacing has to be spot on, and you need a machine with a vice specifically designed to handle those types of keys. For me, I find it to be a really good balance between security and availability/affordability (since I'm not interested in getting a contract for a restricted keyway with anyone at this time).
The downside of the above is "less common" does translate to harder to find and more expensive, and there have been times I've had a hard time finding a particular part I needed at a given time. Also, if you're doing this for a client, the keys are quite a bit thicker than most people are used to, and you may get some complaints from some users on having to carry them on their ring, especially for people with keys to several places in the system.
As Jeff pointed out, the A-R are the standard keyways that have been around for ages. They're more common meaning less expensive and easier to get. Keys are also of a more typical size. That said, they're still actually pretty secure. There are a lot of keyways available so there's less chance for system overlap, especially if you have an idea what keyways some of the locksmiths in your area tend to use and choose a different one. The tight tolerances of the system make them difficult locks to pick, and even though key blanks will be more readily available, many of the keyways are still fairly paracentric and you really need a good, properly calibrated machine to duplicate a working key. In other words, it's highly unlikely that any of your system users could walk into a big box store and walk out with a working key, even if they have the blanks. They could do that at a locksmith shop, but most locksmiths recognize SFIC keys as likely a part of an institutional keying system and may not be keen to copy them for some random Joe off the street. There's a thread on this forum
here with pics of all the keyways, if I had my choice I'd pick something other than A because as Gordon says, it gets used everywhere and there's more possibility for overlap. Go with one of the higher letter and more paracentric ones to avoid that and also make picking harder. E, F, H, N, etc.
