Of course, that 'continuous' rating is nothing like what you'll actually be using during the vast majority of music or movie listening. Most of the time it will likely be putting out one wpc or less, but that will quickly grow with dynamic peaks.
What about 200 wpc vs. 120 wpc? Assuming those numbers are taken at the same THD level (seems unlikely, that .2 Alan posted is kinda high for ratings of this sort), it equates to a maximum volume difference of less than three decibels at one meter from the speaker. It gives you a little bit more capacity to avoid distortion when the cannon goes off during 1812, or during massive explosions on a movie soundtrack.
In almost all cases, the best upgrade you can make to your system is better, flatter, more sensitive speakers, or more subwoofers.