Can't say I've used an Ergo2, but I've tried out a couple others and I own a Ranger and Grip A-Series. I'm kind of a bag whore, and was super intrigued by the Ergo2.
That said, I do prefer the Grip over every other bag I've tried. The quiver pocket is pretty crucial for me, I love the ease of taking out my putters without having to take off my bag. Whereas with the Ranger, it's not as quick taking out and putting my putters back in. The putter location doesn't look as convenient to me on the Ergo2.
Another thing I really appreciate about the Grip is the fact that every slot/pocket-area on the front of the bag, whether intended or not, can hold at least a disc or two in it and fits as if it was meant for that purpose. Whereas the ranger has a huge storage area up top, it's obviously not specifically meant for discs and stuff is just flopping around in there. Doesn't seem to be a huge deal on the Ergo2 though, as all the front space is utilized in the vertical disc storage.
Another thing is aesthetics, branding aside, the Grip-style look looks the most like an actual backpack to me and the least visually obtrusive.
All that said, I honestly really dig the way the Ergo2 looks and honestly might still cop one even with how happy I am with the Grip...
Basically this post probably helped none...