I think it is quite simple. If you have a 1.6x crop camera, then get the 17-55mm. If you have FF camera, then get the 24-105mm (think it comes as a kit for FF cameras).
I won't fuss too much about the build quality of the 17-55. It is built decently. Not L quality, but very decent. In fact, I prefer it this way. If it were built like an L lens, it will be heavier and costs more, while the optical quality probably cannot improve further because it is a damn good lens already. Also, I don't like to use the lens hood. I use my left hand to block out the sun on those few occasions I need to. Hence the 17-55 is perfect for me. But if you need a lens hood, then just buy the 'cheong' version. It is very cheap and pretty much does the same job.
If you compare it with the Nikon 17-55, the Canon one has a far better value proposition. The Nikon lens is built solid as a tank, costs more, comes with the hood, etc... but the optical quality is not as good.
So... everyone is asking... what if I upgrade to FF in the future? If you need FF, then upgrade to FF now. Why wait? The 5D MkII is such a well made FF camera, why bother with the the 7D, 60D, 550D, etc... if you need the FF capability? I choose a 1.6x camera because I like the size and weight of the 550D, and I am happy with the 1.6x crop sensor's capability. FF is nice, but not necessary. If it is not necessary now, it is unlikely that it will become necessary in the near future. Plus... sensor technology will continue to improve. When they make a better FF sensor, they will also probably make a better 1.6x crop sensor. If I am happy with the 1.6x sensor now, I will be happier with the future version. In short, only you can decide what you need... and if you cannot decide which camera you need, then you probably also cannot decide which lens you need.
But I have a suggestion... when in doubt, go for the cheaper option.