I rode this Saddlebred for the first time today, and I would say they are on the more high-strung side. She was great. Sensitive, and alert, but not over-reactive, and lot's of go. Oh, and smooooothhh gaits. Very comfy ride. I have to ride her more, but I think Saddlebreds are a great breed. It's hard to determine the temperament of an entire breed off just a few horses, because there are so many variables involved.
I have ridden Appaloosa's, and the ones that I have ridden have been great. I would consider buying one, but they seem to be hard to come by in these parts. The Appy's I have ridden seem very quarter horse like in temperament.
Finally, I can say both breeds have a lot of history, so you should find some very good information about them in books, online, etc.