According to recent studies, neither of those are really happening. "Secularization theory" (https://en.wikipedia.org/wiki/Secularization#Current_issues_...) is the current sociological term for (a) and it is pretty well established that while there may be regional fluctuation in the numbers of religious people, at a global level, religiosity is not in decline.

As for (b), at least in the US, it is the large mainline denominations that are in steep decline and the smaller, more theologically conservative denominations that are growing most rapidly. Mega-churches are a phenomenon, but not one that falls easily into either of the above categories because they can range all the way from progressive to prosperity gospel to theologically conservative and nearly anything in between.