Definitely navionics. Absolutely amazing for the price. 10 bucks for the whole west coast of BC. Great information and quite usable. I've got a few requests but for 10 bucks... you certainly can't complain.
I took my iphone out for a bit of a test on my last trip out on the boat. The charts are great (The tide chart is awesome), but I was unhappy with the GPS. It did a poor job of showing where I was. It's not a chart problem, as the maps function put me in the same place...
My girlfriend's iPhone is a bit off on the GPS part too.. not sure what's with that. I have the 3GS, maybe it makes a difference? But it never had an issue of pinpointing me in the exact spot I was in.
One really cool feature of navionics is that you can take a picture and it'll tag it in the app. So you can go back over where you were, or just keep a log of what an entrance to a harbour looks like etc and be able to look back on it later or another time you go there.
Bought the Navionics on the weekend and played with it a bit. You're right, you can't go wrong for ten bucks for the entire Great Lakes including the St. Lawrence. The GPS in the 3GS is bang on with placement, even put me on the right part of the slip where I was sitting.