I have been using a number of different types of brushes over the years painting miniatures and undoubtedly the biggest improvement to my painting came from investing in a quality brush; when you mention quality brushes two names invariably get raised - Winsor & Newton Series 7 and Raphaël 8404.
I have a confession to make, I am historically biased having used W&N Series 7 for the past decade or more as they were easier to get hold of and produced in the UK (got to support local industry!), however recently I have tried the Raphaël and fallen in love.
Both brush types have a similar cylindrical shape, bulging towards the ferule, however it s immediately apparent from handling that the Raphael 8404 brushes are a little bi narrower which is definitely noticeible with the smaller sizes (0-000). Depending on your hand size this may immediately make the W&N more comfortable to use, however for my average paws both are more than comfortable for a typical 3 hour painting session
The Series 7 has slightly more hairs at the Ferrule, however the 8404 has a wider body with longer hairs which provides a larger reservoir, holding more paint and keeping it wet for longer which I have found to be a real benefit since using them. Of the three brushes here the W&N Miniature brush has short hairs which provide the ability to paint clear detail but sacrificing painting time as the shorter hairs lead to the paints drying much faster. The hairs in the 8404 seem to be more coarse and have more spring and whilst the body is wider, the point is excellent and even using a size 1 brush (the equivalent of a Series 7 size 2) I am confident in painting most detail.
When it comes to price both ranges are high quality hand made brushes which costs a premium vs. a basic brush you can pick up in a supermarket or hobby store, however with the right care (see my brush care guide here) these brushes will last many hundreds of painting hours. 8404 are slightly cheaper than Series 7 (typically up to £2) which may sway your choice, especially as the quality of the brushes is near equal with the brushes having different qualities that will appeal to different uses.
You ultimately can't go wrong with either range of brushes, I would advise to go for the standard W&N Series 7 over the miniature range for most uses as the shorter hairs lead to the acrylic paint drying far too quickly. With the miniature range out of contention what would I recommend? As mentioned I have used W&N for many years and find them hard to fault, however the past months using the Raphaël 8404 have been a real eye opener. For blending the longer, coarse springy hairs keep the paint wet for longer and still produce the detail required and for me, including the difference in price get my recommendation. However, I would ultimately recommend trying a size 1 (the size I typically use most in painting miniatures) in both ranges to see which you get on with best.