Okay, now were talkin'! I have an iphone too, and the garmin 305 replaced it, so I don't use the iphone anymore. The only downside to the garmin 305 is that even when using the gsc-10 speed sensor, the instantaneous speed is off at times... max speed is always 1mph slower than another computer because it relies on satellite data even if the speed sensor is hooked up. annoying. I have the powertap computer there too so its no big deal to me because I have a better instantaneous display. Average speed is perfect, and I would imagine it works better when your not in the woods, but I ride in heavily treed areas for the most part. I am not sure if the 500 is better in that way, but I would get the 500 if you are ever going to think about a power tap. Other than the annoying displayed speed issue, I love the 305, though.
I know nothing about the polar but I have read awful things about how you have to download the data with a microphone or something goofy like that. I like their products a lot, but that was a no go for me with the computer. I dont think I was looking at the 500, though, so make sure that info is accurate. If it has wireless or usb downloading I bet its pretty good.