It is a very close call and in one sense tossing a coin would be as good a method as any in deciding. However I would go for Flavel. His "Fountain of Life" is just brilliant as is the "Mystery of Providence" as Wayne suggested. I would go for Flavel this year and at the first opportunity buy Sibbes later.
It's kinda like asking which 10K diamond do you want - the round or marquise cut? Can there be a bad choice when those are the options?! But I have Sibbes and love him dearly. So I say Sibbes!