Samsung says they're on track to selling 30 million Galaxy S6 units by the 26th day of release.
http://www.cnet.com/news/samsung-demand-for-galaxy-s6-steeper-than-expected/
It seems like both Apple and Samsung interpret shipments as sales.
Considering that many carriers are out of stock with the original shipments of Galaxy S6 units (20 million) and Samsung claiming to ship another 10 million by the 26th, 30 million sales are likely by the 26th day, which could outpace the iPhone 6 (don't know for sure because Apple never releases sales figures for specific models).
Apple sold 74 million iPhones (including the iPhone 5, 5S, 5C and 6) in Q4 2014. Probably half of them were the iPhone 6 which means 37 million units of the iPhone 6 were sold in 3 months.
Q4 of 2015 will really determine which company has got the upper hand as the iPhone 6S is coming out.
The iPhone 6S must surpass iPhone 6 sales or else I think Apple will be in big trouble because it would be their first ever iPhone sales decline.