Correct, cost is the only significant factor.
However, more watts = more headroom, the unit will run cooler, as it ages you won't deal with it no longer being able to support your components, more upgradability, etc.
For the 80+ Bronze/Silver/Gold/Platinum etc. I'll try to explain.
The rating system is how efficient the power supply is. Efficiency being the relationship between input and output. A more efficient power supply will lose less input as heat than a less efficient power supply. For example (all numbers made up in this example), if PSU-1 is 80% efficient, and PSU-2 is 90% efficient, and your components draw 800W of power, then PSU-1 will have to draw 1000W from your wall to provide your components with 800W of power, where PSU-2 would only have to draw ~889W. So, over time, a more efficient unit will pay back in your power bill.
However, if you only run your computer a few hours a day, this may take YEARS to pay for itself, if it ever does. The more often you use your computer, the more attractive a higher-efficiency unit will look. 80+ Bronze, 80+ Silver, 80+ Gold, etc. just refers to different certified efficiencies (tested by 80 Plus).
80+ means the unit has been tested to 80% (or higher) efficiency at 20%, 50%, and 100% loads.
80+ Bronze = 82%
80+ Silver = 85%
80+ Gold = 87%
80+ Platinum = ...92%? I think