I was curious if it makes sense acquiring a 1" angled Purdy brush just for cutting lines. Currently, I use a standard 2.5" angled brush for corners and I'm not too good at cutting lines with it.
I would think a 2.5" brush gives you better control, because a thinner brush can result in a more wavy line. With a larger brush, the "waviness" is controlled a little better.
And, a smaller brush can only hold so much paint, and you'll end up with more stops and starts due to very nature of the thing. That can introduce some more imperfection in the process.
Also, you'll end up holding the brush for much longer, and if you're anything like me who can't hold a pencil for longer than five or ten minutes before needing to rest, the entire process will take much more time.
Of course, your mileage may vary.
The 2.5 inch angled brush is ideal. My favorite is Wooster's Silver Tip. It fits your hand better and holds more paint, which allows your strokes to be longer and more consistent. The small brush requires more stops and starts, where your lines will be less straight.