The detail brush from GW is adequate for most stuff, the trick is just how much you load the brush. I love the detail brush because the belt thickness on Cadian guard is just right to use the brush just to go round in one stroke and get good coverage.
While I find it... difficult - but by no means impossible - to do the very fine detail such as tricky highlights and good lens effects on small items or very small metal studs and clasps etc. this brush is still my go to brush and works well so long as I use very little paint on the brush.
However, I too still searched for a smaller brush and decided upon the Army Painter thinnest brush. I havent had a chance to use it yet but the size difference looks to be enough to make it a worthwhile purchase.
I've got to say I'm new to the brand. I used the store to get their sand yellow primer for my guard, but due to the weather and work haven't had a chance to use it yet. As far as my hearsay web based experience goes most people use them for these primer sprays but the serious painters will get their standard acrylic from elsewhere, such as www.acrylicosvallejo.com/gb/model-color-gb.html
So I will leave the rest for other members to answer.
I don't bother with GW brushes as I find them poor quality for the price you pay - the "fine detail" brush is not fine enough and none of them survive a decent drybrushing session. I tend to use Humbrol or any other dedicated crafts brushes in my local (non-GW) model shop.