The two books are very different from each other, however. Wolf Hall is certainly the better of the two: In Mantell's hands Cromwell becomes a fascinating character, carrying the baggage of a brutal London upbringing, always mindful of his family away from court, ascending the greasy pole of power rather in spite of his own best instincts. She really summons up the smell and feel of Tudor London, and the alarming sense of fragility of life - not just from the king's displeasure, but from illness, violence, or accident. The novel ends with Cromwell's ascent to full power; I believe a sequel is brewing which will cover the last five years of his life, and I will certainly buy it.
The Other Boleyn Girl is, alas, a fairly standard romance novel with well-known characters. Unlike most historical accounts, Gregory makes Mary the younger sister, watching as the older Anne first connives at her own affair with the king and then ruthlessly replaces her; as Anne approaches her doom (which she partly brings on herself by incest and witchcraft), Mary ends up with a nice but humbly born man who takes her away from it all. Mary is such a naive first-person narrator that it gets a bit irritating at times. But it is well-written and perhaps more approachable than Wolf Hall.
The central character of each of the two books is a background figure in the other, and neither is particularly well served. For Mantell, Mary Boleyn is a fading but demanding former royal mistress with important but fraying family connections. For Gregory, Cromwell is a dodgy political figure ensuring Anne's rise for his own reasons. Both of them come out in roughly the same place in portraying Henry VIII as randy, short-tempered, and tough on his advisers (Jonathan Rhys Meyers' portrayal on TV is of a rather younger man).
After reading these, the one person who I really ended up wanting to know more about was Anne Boleyn. Only Mantel explores her character at all positively - she is the villain of Gregory's book, and the depiction of her as the court flirt in The Tudors goes back at least to Shakespeare and Fletcher. But she kept Henry chasing her for years (from their first encounter in 1525 to their marriage in 1533), which is pretty impressive considering that he could basically have had any Englishwoman he wanted. It's also strongly suggested that she was genuinely Protestant in sentiment, which would make her a rather advanced thinker and would perhaps give her an extra motive (besides the obvious personal one) for wanting the Church to be under direct royal control.
Anyway, we have another 33 episodes of The Tudors to go, if we can keep the pace.