Hummm.. well I personally have never been a fan of H/K . The best way to go though is to listen to both side by side if you can and then pick. Is there an A/V shop near by that carries both so that you could do that? I'd lean towards the Marantz but I haven't auditioned that model.
I could audition both, but they would be on different speakers at different locations. Is there any reason you havent been a fan of H/K?
I'd look into the better-built Marantz 5400, it's recently discontinued so you should be able to get some sweet deals on it, probably close to the normal retail price of the 4500. Maybe even less if you're willing to buy refurbished, B-stock, or a floor model.