I'm a Milwaukee guy through and through, but I'm pretty partial to Metabo for anything-grinding. They're pretty much unquestionably the best.
HOWEVER, for cordless, I really think the Dewalt Flexvolt grinder is the one to get. It's the only cordless 4.5-6" sized grinder that genuinely surprises people with its power. Look up any review. All brushless grinders generally get the "it's pretty/plenty powerful" stamp, but only the Flexvolt gets the "WOW I can't believe this isn't corded" stamp. It's also really ergonomic. Go hold on at your local HD. Priced very well with the standard 2Ah packs and you can always get the 3Ah bad boys with the bigger/better 20700 cells. I bet it's a mean machine with one of them...
I'll say this: I have (11) M18 battery packs, the majority being 5.0s and 9.0s. The last thing I need is another battery and charger, let alone different ones. I also don't currently own a cordless grinder (I'm pretty hard on grinders). If I needed one today, I'd pick up the Flexvolt.
The only thing I'd be weary about is them dropping the battery platform since they've more or less pioneered that game. Though for some reason, I don't see them doing that with this one...tons of potential with the higher voltage. It's really pretty clever.
If "good enough" is fine with you, get the Milwaukee Fuel grinder. A huge plus there is the M18 platform (several class-leading tools). Good luck.