I've only used the Garmin Forerunner 305 for a couple of times (25 miles) and don't own an iPhone, but I've been running with an iPod touch for the last 4 months and I've found a huge difference.
iPod touch (which can be extended to any iPhone) aren't very suitable for running long distances. I've been wearing it in an neoprene arm band (Incase) and the inside usually ends up building moisture, sometimes starts malfunctioning, the Voice Control feature pops up, tracks are played at faster rate, the player leaps randomly between tracks... what it can become very annoying. Fortunately, when the moisture goes away it woks again normally. In contrast, Garmin 305 is certified to work under 1m of water for half an hour so moisture or even heavy rain shouldn't become an issue.
On the other hand, the Garmin 305 has very nice features for any runner curious about his/her runs. I've checked the registered distances in Google Earth and they're rather accurate. You can set to register your position every second, however in the long runs this feature is automatically deactivated to store more data, this explains why in such cases your route appears with much less points and more often goes through buildings and out of your pathway, however I've verified (comparing short laps that have been affected by the "smart simplifications in the map with the real distances) that the measures are still made with the same accuracy.
Another reason o wear Garmin 305 is the use of foot pod to track your run cadence. I've been running barefoot since February and was very interested to check my cadence to keep it high (at least 90 spm), the most recent foot pod model is as small as the Nike+ sensor but unlike the Nike+, Garmin foot pod uses replaceable batteries. The cadence readings are rather consistent despite the fact that I have to wear the foot pod attached around my ankle.
As for the Garmin 305 size, the watch is rather big but isn't heavy and the wrist band fits to very narrow wrists like mine. You'll forget you're wearing it.
The last extra feature that iPhone doesn't have is the heart rate monitor, if you find uncomfortable the included chest band, get the Premium HR monitor, the transmitter is attached to the fabric band by means of 2 snap fasteners that also are used to connect it to the electrodes. The strap itself isn't of very good quality but interestingly the transmitter fasteners are fully compatible with the Polar bands such as Wearlink + transmitter for Nike+, so if you have been using it with your Nike+ device you can still recycle the strap to attach the Garmin Premium transmitter to it.
Finally, you can to convert your Garmin workouts to Nike+ with a free online application (
http://awsmithson.com/tcx2nikeplus/), that way you can see the page that has been created to depict routes for the new Nike Sportwatch GPS in the Nike+ site (compared to the Garmin site it looks crappy) and how slow is loaded. Two examples of Garmin activities:
activity1,
activity2
Edited: 18/06/2011 at 19:38