I've never had any dealings with bears, so I can't say how the XS sights would do for that purpose. I have put a lot of rounds through my 325NG and I carry it regularly.
For me the sights work very well. My eyes are not what they once were and the big dot really helps. I have noticed that I need two different sight pictures, depending on range. For close range, say 20 yds or less, I place the dot on the desired POI. When distances are much greater than that, I use the top of the dot as if it were a post. Sounds more complicated than it really is. Like most things in shooting, the more you do it the easier it gets.
As for the big dot vs little dot issue, I've never had an issue with a lack of contrast. The sight has some contrast built in because of its "donut" configuration and that helps. I can't say that you would never lose the the sight because of background/light conditions, but that is a possibility with any sight system. Think putting a black sight against a dark grey hog at twilight.
For what it's worth.