It's overkill. If you already have the iPhone, then you just need a basic GPS. You already have all the bells and whistles, why spend a ton of money on more bells and whistles!
Unless you don't care about the money. In that case, I'm sure the Oregon is just fine.
I can't say that I agree that it's overkill as it depends on your circumstances, though I would agree that there would be plenty of doubling of resources.
But first, I'm not sure I'm clear on the core question:
If the question is iPhone / GPSr, then there is already plenty of discussion in this topic and I'd suggest reading all the posts
If the question is which GPSr, then a search of the forums for a more related topic or posting a new one might be the best way to go
Thanks for the replies. I think i have just saved some $$$.