Shaman,
Flat base bullets are inherently more stable in flight than boattails. So, in calm conditions, for ranges out to a couple hundred yards, in general you can expect better groups from flat base bullets.
Boattails have a higher BC, which at longer ranges can off set the advantages of the flat base when you look at a total Circle of Error Probability (CEP). The exact range where this trade off occurs depends on the differences in BC, but in windy conditions, the additional BC can make a huge difference.
Like most shooting related decisions, it's a trade off and each has to decide what best applies to his hunting/shooting conditions.
