There are no studies that compare "seeker sensitive" megachurches to small "missional" churches, but I think Dan Kimball is right to question the self-described "missional" advocate who declares that "younger people in the city will not be drawn to larger, attractional churches dominated by preaching and music."
The evidence shows that more and more people are attending large churches. Duke sociologist Mark Chaves writes, "In every denomination on which we have data, people are increasingly concentrated in the very largest churches, and this is true for small and large denominations, for conservative and liberal denominations, for growing and declining denominations. This trend began rather abruptly in the 1970s, with no sign of tapering off."
Furthermore, the 1,250 megachurches in the US in 2007 show remarkable strength across a range of indicators, according to Hartford Seminary sociologist Scott Thumma and Dave Travis's Beyond Megachurch Myths. Thumma and Travis take seriously the stereotypes of megachurches as impersonal, selfish, shallow, homogenous, individualistic and dying but they do not find the accusations match the data.
Even Baylor sociologist Rodney Stark's What Americans Really Believe lauds the strengths of megachurches as compared to small churches. "Those who belong to megachurches display as high a level of personal commitment as do those who attend small congregations" (p.48). This is significant because some of Stark's earlier work claimed growth dilutes commitment. In 2000, he declared, "Congregational size is inversely related to the average level of member commitment . . . In all instances, rates of participation decline with congregational size, and the sharpest declines occur when congregations exceed 50 members."
Furthermore, the Willow Creek Reveal (2007) and Follow Me (2008) books, which some were led to believe denounced the seeker megachurch model, provide zero data about how different sizes of churches fared on their surveys. We have no idea whether small or large churches like Willow report more or less "stalled" or "dissatisfied" people than others.
But megachurches are not the only ones thriving. Many new churches are being planted, and many of those would describe themselves as having a "missional" mindset. David Olson reports that in the fourteen diverse denominations he studied, all the denominations that were growing were planting lots of churches; specifically all those denominations planting at least one new church per year for every one hundred existing churches continued to grow. The denominations also range between a 52 and 88 percent survival rate in new churches. First year attendance ranges between 44 and 145 (Olson, 149). In 13 out of 14 denominations, new churches are growing steadily (Olson, 150). The point is that though megachurches are continuing to thrive, new churches (often "missional") are also a very effective part of the American church.
I suppose one could do a survey of a number of self-described "missional" churches and "seeker sensitive" megachurches and see where they rank on a battery of criteria: adult baptisms, attendance growth, as well as the core Christian beliefs, practices, and virtues. Then one could do a study five years later and try to discern the "effectiveness" of "missional" versus "seeker sensitive" approaches. But even if we did such a study, most pastors would rightfully stick to what they perceive as "working" in their own community. Perhaps that is the key insight that powers both missional and seeker sensitive churches: Churches should adapt their forms to reach the people in their community (while retaining faithfulness to Jesus).