I think the main differences you will find are that the 60D will be a better build, I believe the 60d uses a prism on the viewfinder rather than a mirror which should give a brighter view through the view finder, it supports more custom functions than the 60d and has a faster burst.
Another thing the 60d does have is a mode dial on the back which makes it quicker to operate in manual mode as you can control the aperture with that wheel and the shutter speed with the front wheel in other creative modes you can use it to control exposure compensation.
I guess it really depends on how much extra the 60d costs over a 600d and whether you can justify the extra cost and if you would use or hanker after the extras on the 60d.
Personally I would try both at a local camera shop and see which you prefer.
BTW is this your first DSLR or are you upgrading?
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