Hi.
I tried both in a shop taking pictures down the high street. Image wise the prime was better but it was a nice bright day so I had a fast shutter speed.
I eventually went for the zoom because it isn't always a bright day and woods are shady and with the slower shutter speed the IS comes into its own especially when hand holding.
My next big lens would be a prime because of he better image quality, something like the canon 500 f4 is, but thats a few years off yet.
Cheers David