Angle grinders are something of an odd ball in my tool collection.
I generally recommend people who purchase battery based tools to stick with one brand in order to best take advantage of a single source of batteries instead of having a bunch of miscellaneous chargers laying around. HOWEVER, with angle grinders, consider how much use it is going to get. If you are going to be using it quite a bit, I lean towards getting a corded version so I don't experience power drop off and limited runtime.
I utilize
Ryobi 18v tools so I would purchase their version of an angle grinder due to that. Some millwrights that I know swear by Metabo grinders.