Can't speak for everyone but I just don't think they're equal on talent. I think Stirzaker gets to the breakdown quicker, takes better options when he gets there, has an equally good running game and a much better pass. He's not quite as big yet, and possibly not as active in defence - but the flipside to that is he doesn't miss nearly as many tackles. Phipps is just okay for mine. He's got an alright pass, a good but predictable running game, an average kicking game, takes decent options etc. but he hasn't really kicked on. I've never seen him change a game or really own one. I talked about it earlier in this thread, but see if you can get your hands on a copy of the Rebels-Stormers game from earlier this year. The entire Rebels backline sparks up when Stirzaker comes on.
As for Owen, he was found out against the big boys I think, had a nightmare in that Stormers game for one.