The choice is largely based on personal preference and what type of paint you're using. Most bristle brushes really aren't suitable for water based paints such as emulsions, acrylics etc, because the bristle tends to absorb water from the paint, which as you can imagine really isn't that convenient. So for water based paints I would use a nice synthetic brush with a chisel edge.
I tend to use a 4" (100mm) synthetic brush to cut in, I've moved away from the angled styles and stuck with a nice straight cut with very fine bristles. I find that size allows me to cut in quickly and efficiently as the bristles tend not to move so much; simply because there's so many of them. The synthetic brush I use has very fine strands so it flows out the water based paints very nicely so they blend with the rest of wall perfectly.
What I will say about natural bristle brushes is that they come into their own when they've been worn in. The more you use them the more supple and chiseled they become. And remember it's not all about the price of the brush, more the quality per £. You can get imitation Hamilton's and Purdy's which are a fraction of the cost and do a very good job with the right paint. Which reminds me that I once proved I could get a brush mark free finish with a fence brush, you know those 4" rough strand brushes, that was all down to the paint and the thickness applied, but I did it and not a run in sight.