I am using Newicons for more than 10 years and i love them.
Don't know much about Glowicons, but i think they are compatible with newicons.
I belive there is some confusion here. First of all, GlowIcons is an icon style and not an icon format. The icon format used by AmigaOS 3.5/3.9 is called ColorIcons. It is possible for AmigaOS 3.5/3.9 to use NewIcons in addition to ColorIcons, but AmigaOS 3.1 and lower can't use ColorIcons.
I do belive there are tools on Aminet that can be used for converting NewIcons to ColorIcons and vice versa.