It seems to me quite likely that in many cases Wikipedia articles will be more accurate and certainly more indepth due to the sheer number of people editing (potentially anyone who has an internet connection)

Also I think due to the number of editors there are more likely to be people who have a specific interest or experise in the subject matter editing any given wikipedia article than the editors of Britannica, who I would imagine have a more overall knowledge of the subject area they are allocated to.

Of course the downside of wikipedia is that anyone can edit it and write complete rubbish, although my experience of it is that there are so many people using wikipedia that inaccuracies tend to be corrected quickly.