I like that one on ebay with the dial thermometer right on it, if you could get that at your desired price, that would be my pick.
I have a thermostatic valves have the advantage of putting out a set temp, while a tempering valve will mix based on a set valve position. variations in water input temp will vary the output temp for most valves of this type.
I guess the bottom line is it depends on what you want to accomplish (AKA requirements
). If you read up on the design characteristics on each it will probably become clear which type would best suit your needs.
See how much time these "little" science projects can consume?! But just think how much you are learning.