I think it also depends on what your painting style is. When I want to do loose style with lots of water (mainly landscapes), I love a bigger round sable brush and use my Rosemary&Co’s Pocket pure Kolinsky sable as well as the squirrel mop, as they hold so much water. They are *very* difficult to get back into their caps dry without damaging them though, so I tend to put them away slightly damp and they dry in the cap thanks to the small hole in the bottom of the cap.
However, my personal favourite painting style is realistic, and for that I need a good point, a brush that’s not too soft and a lot of snap – a smaller synthetic usually. In this case, I think my favourites are the DaVinci Cosmo Top Spin (synthetic) travel brush 1573 series. I have a size 4, 6, 10 set that I use almost every day. They may not hold as much water as the sables, but they have served me well and are my favourite travel brushes by far. I also really appreciate the fact that the top cap’s hole is bigger than some of the travel brushes I’ve tried, making it easier to insert the brush without messing up the hairs (I just wipe the bristles gently with a paper towel and they insert easily, keeping their shape).