Is a snap on crowfoot open wrench with a 3/8" drive extension a good tool to use for valve adjustment?

Should work, that is basically what the special tool is anyway. But it is a good idea to double check the lock nut with a wrench after you are done as sometimes you will find using this tool you don't get the locknut tight. You can't get the right leverage on the tool to tighten them as tight as they should be, they snug them down good to keep the adjustments, but it is better to be safe and double check it before you re-install the valve cover.

Is there a better tool to use than the crowfoot that "snap on" offers?

Not that I know of, in fact the valve adjustment tool that MSA sells is in fact made by Snap-On, that may be why it's so expensive. It is a type of crowsfoot wrench but is made on one piece.

Take a look at it if you have their catalog and you could try to catch a Mac Tools salesman or Craftsman may have one very similar that will be a bit cheaper.

BTW, I have used just 2 plain open end wrenches to adjust valves, but they tend to get in each others way. Unless you have a cheap one you could put in a vise and bend just a little so they aren't interfering with each other all the time.