I swapped to a Suunto Ambit from the iPhone mainly because I got fed up of always having the phone with me on the bike (I also use it for walking). On a longer ride I tend to take my phone anyway just in case I have to call the team car (wife) in the case of mechanical failure but if I'm just going for a quick blast I prefer to travel as light as possible. This time of year the phone is fiddly to operate with gloves on and if I'm on the mountain bike I tend to be muddy (although I do keep it in a waterproof wallet). Real time data is available easily, all the time on the watch.
Battery life is also an issue. The Ambit will do about 16 hrs (sampling GPS every second) but can go for days samping every 60 seconds. My iPhone running Strava isn't bad but I don't think I'd get much more than 8 hrs from it. I also like that the Suunto will record my progress whilst giving navigation information for routes that can be uploaded to the watch. I didn't have it at the time but this would have been very useful when a did a night time sportive before Christmas that had useless directions on the road.
In terms of fitness, watches like the Forerunner have training functions that you can adjust to suit your training goals. For example, you can race against a virtual partner and have the watch alert you when you fall outside of set lmits (heart rate, pace etc). Not sure if there are phone apps that do the same thing real time as you are exercising but it's much easier to glance at a watch than dig your phone out of your pocket.
Take a look at the reviews on this site. The guy does excellent in depth reviews of fitness devices and it will give you a feel for what they can do.
http://www.dcrainmaker.com/