I've done a bit of searching and was unable to find what I expected to be a simple statistic. By "efficiency" I mean the percentage of the Plutonium in the bomb that actually fissioned.
According to the Wikipedia article on Trinity, the estimated yield of the device was between 18 and 20 kilotons from a 6.2 kg plutonium 239 core. Complete fission of Pu-239 yields 19 kilotons per kilogram, giving an efficiency of somewhere between 15% and 17%.
For the Hiroshima bomb (Little Boy) with 64kg of Uranium only 700 grams was fissioned, or about 1.09% before the remaining HEU was blown apart.
Interestingly a soil sample survey from Hiroshima in 1983 could no longer detect evidence of Uranium above background levels.
The superiority of using Plutonium came from the fact reactors at Haniford could produce it from un-enriched Uranium. It also made the weapon fail safe.