Once I get technical in this little head of mine, I think that the throttle servo being stood up has the same benefit as being laid down. If you think about it, all the weight of the servo is toward the bottom. Also, to get even more technical, when you lay the throttle servo down, you are spacing it over a larger area of the chassis, making it "hang off" more, which, to the nitpicking mind, will give more polar movement. Also, I have never had a car with a laydown throttle servo, but I can imagine that the linkage is a pain.

When it comes down to it, I don't think there is an advantage either way.