I use as cheap a brush as I can get away with. Expensive brushes are nice but I tend to leave them out too long. I try to clean them but they are never the same.
I can often find pretty good brushes in house paint stores. There is a Sherwin Williams near me and I got some great and cheap brushes there. They are used for detail work in house painting and they work great for general painting work. I think they would work equally well for oils and acrylics. I’ve seen similar brushes in other home painting and hardware stores. Sometimes the ferrule is a little loose but I don’t care so long as it holds paint.
I tend to use oil painting brushes because they are often stiffer and that works better for me. Maybe because I started years ago with oils and have only recently switched to acrylics. I like Filberts and rounds but use whatever seems right at the time. I have a good collection of old brushes that are often helpful. Fingers can work just as well but not my style.
Here is a picture of my present brush collection. These are only the ones I presently use because I recently went through them and threw out all the really ratty ones. I think the trick is to get paint on the canvas. The way that you get it there doesn’t much matter.