You wont notice much difference between say, 20W and 50W in most scenarios, most of the time. The extra power might help penetrate through buildings and things like that, but, most of the time, on average, there is not a huge, if any difference.
One example: Just recently I was testing a UHF mobile radio (XTL5000 for those interested) operating just above the GMRS frequencies. At about 90 miles, I was able to hit a repeater at 50Watts with a signal of about 3 out of 10 on the receiving end. Bumping the power up to 110W, made no audible difference in my signal strength or quality.