The A-Theory of time is the most widely accepted of the two and for good reason. As philosopher William Lane Craig explains:
"According to A-Theory, things/events in time are not all equally real: the future does not yet exist and the past no longer exists; only things which are present are real. Temporal becoming is an objective feature of reality: things come into being and go out of being."  This is the commonsense view of time. Past events are no longer, the present is real, and the future does not yet exist.
In contrast, as Craig explains, on the B-theory of time, "...all events in time are equally real, and temporal becoming is an illusion of human consciousness. Pastness, presentness, and futurity are at most relative notions: for example, relative to the persons living in the year 2050 the people and events of 2000 are past, but relative to the persons living in 1950 the people and events of 2000 are future. Things and events in time are objectively ordered by the relations earlier than, simultaneous with, and later than, which are tenseless relations that are unchanging and hold regardless of whether the related events are past, present, or future relative to some observer."  On the B-Theory of time you can think of all events, past, present and future, as represented on a yard stick. We are right now somewhere on the yard stick, but all the events represented by the yard stick are equally real.
For those interested in learning more, I recommend this short video in which Dr. Craig explains the A-Theory of time and B-Theory of time and how it relates to the Kalam cosmological argument for God's existence.
Which theory of time do you hold to? Please share in the comments!
Courage and Godspeed,
1. William Lane Craig, Reasonable Faith 3rd. Ed., p. 121.
2. Ibid., p. 121.