I hear that it's IQ of the 17-55 IS is similar to the much cheaper Tamron 17-50mm which goes for around $450 or half the price of the 17-55 IS.
If IS really means that much to you I'd go for the Canon, but otherwise the Tamron is fine, I have one and it's IQ is superb but I'm finding out that IS really would be more of a benefit for the shooting I do (inside and low light) so I'll upgrade in the near future.
Even if you plan on full frame in the future, the 17-55 will hold it's value so don't be too worried if you purchase it now and want to purchase a FF camera and lens later, someone will buy the 17-55 if it is well kept in the future, these days it goes used for around $850-900 compared to the retail price of 949.