ok couple of questions... if you are looking at used bikes, how do you know you are going to be changing gears several times a minute?
Why are you changing gears so often?
now that the questions are asked... if you actually change gears that often, down tube or stem shifters would mean you will be riding with one hand off the bars all the time...so if you are actually going to be changing gears that often you would need to have STI shifters.