The advantages both formats provide is the chance the group has to visit with each other over a meal. This becomes an excellent way to establish a sense of community and a more relaxed and informal atmosphere. The limitation of both Alpha and Beginnings is trying to contain the whole of the Christian faith into a classroom model, as something that is just taught, heard, and discussed. This shortcoming can be improved by incorporating Christian practices and group experiences outside of the sessions. Why? Because the faith and tradition is not only contained in words, but also in doing it.
Both experiences are intended to be longer than your typical 6-8 week block for short term studies. Alpha stresses the importance of going the distance of 12 weeks plus the Holy Spirit overnight or day long retreat, while Beginnings is a shorter 12 weeks (or 9 weeks with an overnight or day long retreat).
There's a sense in which Alpha asks participants to complete the entire course before making a faith decision for Christ, and, as such, to do so methodically, somewhat logically. Alpha, written in 1993, is more modernist in outlook. That is, it assumes that all things, including Christian faith, can be thought-through and understood rationally. Alpha uses its many Bible references as an evangelical tool to lead to conversion and persuade the participants of the truth of Christ. In this sense, it is not unlike the many Bible tracts of my youth, except on a more sophisticated and developed level.
Beginnings (2003) addresses the post-modern experience that truth cannot always be received just from logical deduction. Its approach is more narrative and invites self-reflection. Its use of the Bible is very different from Alpha in that most sessions use one or at most, two stories from the Gospels or Acts from which to develop the sessions. Whereas, Alpha suggests that the seeker will find the truth from hearing it well presented and then thinking on it, Beginnings emphasizes that we find God also as we engage the practices of faith, such as Bible reading, study, prayer, service, and giving. Whereas Alpha encourages our meeting God through personal decision, the theme of Beginnings is that God meets us where we are.
Although I have used both resources with a variety of groups, Beginnings , in my opinion, is more adaptable in introducing others to Christian faith. Beginnings also offers two follow-up studies: Habits of the Heart (on the spiritual practices), and Around the Fire (Christian community). Both of these are good, though I found Around the Fire, with it's focus on Acts 2, more popular with my groups.
Isn't it great that pastors have a choice?! Third option? Yet another possibilty is Rowan Williams' Tokens of Trust, which is a very readable introduction to Christian faith using Bible and the Apostles and Nicene Creeds. Whatever you decide, I think the best time of year to offer these kinds of groups is following the biggest church seasons, such as Lent-Easter.