The donation efficiency is a query of the Bter PTS-to-BTC exchange rate and I believe its applied to the average daily donations to help you decide which of the two is currently a better projected value. IMO, your best bet it to look at the previous donation trend for each and invest the one which you feel will have the better return.
Just for clarification, that site allows you to receive AGS (angelshares). If your goal is to get as many BTS as possible, donating and acquiring AGS is the best option; however, buying PTS and holding it in your wallet gives you 1.3 BTS for each PTS and you're still able to sell your PTS whenever you'd like whereas AGS is not as liquid.
As for timing, it's not too late to acquire PTS or AGS but you'll need to do so by the end of the 28th GMT.