USB 2.0 will deliver 0.5A on 5V = 2.5 W
USB 3.0 will deliver 0.9-1A on 5V = 4.5-5.0 W (especially in PC, not all notebook/laptop USB 3.0 will deliver 1A)
Just be careful about assuming 3.0 can do high numbers. Even if a single port can, it doesn't mean the array of ports / other electronics or even the PSU can take many many maxed out 3.0 ports. If you tell a consumer that it will work fine on USB 3.0, they'll assume that if one works then they can put 6 in.
Even the 'big' PSUs like the CX750M only have 25A on the 5V rail, of which a proportion will be taken up with other components.