If the SDA do have a bigger membership, the difference would only be by a couple of million at the most. As Slim commented the more accurate size of the JWs would lie between the publisher amount and those attending the memorial. I think the truer JW membership would be around the 16 million mark. If you were to factor in the attendance of those who truly believe the writings of Ellen G White you could also come to the same figure for the SDA. Let’s just take a rough estimate and say they are the same.
The point that really comes home to me with the JW / LDS comparison, is that having more money doesn’t necessarily mean more members. Think on this for a moment. Mormonism had a 50 year head start on JWisim. By the time of the first printing of Zions Watchtower, Utah had been granted statehood by the US government. So Wt in the course of 200 years since the inception of Mormonism, not only closed a 50 year gap, but was able to increase its activity rate by 200%. They also done it on a fraction of the money that both the LDS and SDA has. In the last 20-25 years Wt is certainly in decline, but over the course of its history, with all its scandals and false doctrines, their performance would probably make Microsoft or apple computer green with envy.
I also think Mormon members are more prepared to put up with the nonsense of their leaders e.g fudging numbers, and hiding facts. Could you imagine the outrage, if Wt opened a $5 billion dollar, church funded, shopping complex. While Tony Morris, cutting the ribbon on the opening day said “let’s go shopping.”